Online Material – Tutorials – Papers

# Online Material

- Frank Deppe (WMI) presents QMiCS on the European Quantum Week (https://eqw.qt.eu/), Nov. 2-6 (2020):

- Visa Vesterinen (VTT), Swinging up the quantum signal, QTF March meeting (2019):

- Matti Partanen (WMI), APS March meeting 2020 slides on “Quantum sensing and communication with superconducting microwave circuits”.
- The quantum internet is within reach (Technical University Munich Research News, 24.20.2019).
- An Introduction to Quantum Microwaves for Communication and Sensing (Frank Deppe is interviewed for AZoNano).
- Internet Quântica: mais rápida, segura e sensível, com protótipo em 2021. Portugal entrou nesta corrida através do Técnico, que ganhou dois projetos europeus de €13 milhões.

# Tutorials

- Tutorial on “Quantencomputing” given by Frank Deppe at the Academy for Teacher Training and Personnel Management in Dillingen a. d. Donau (2019-09-25).
- Tutorial on “Propagating Quantum Microwaves” given by Frank Deppe at the PhD School “Cryocourse 2018” at Aalto University.

# Papers

### 2022

Ibarrondo, R.; Sanz, M.; Orus, R.

Forecasting Election Polls with Spin Systems Journal Article

In: SN Computer Science, vol. 3, no. 1, 2022.

@article{Ibarrondo2022a,

title = {Forecasting Election Polls with Spin Systems},

author = {R. Ibarrondo and M. Sanz and R. Orus},

doi = {10.1007/s42979-021-00942-9},

year = {2022},

date = {2022-11-01},

journal = {SN Computer Science},

volume = {3},

number = {1},

publisher = {Springer Science and Business Media LLC},

abstract = {We show that the problem of political forecasting, i.e, predicting the result of elections and referendums, can be mapped to finding the ground-state configuration of a classical spin system. Depending on the required prediction, this spin system can be a combination of XY, Ising and vector Potts models, always with two-spin interactions, magnetic fields, and on arbitrary graphs. By reduction to the Ising model, our result shows that political forecasting is formally an NP-Hard problem. Moreover, we show that the ground state search can be recasted as Higher order and Quadratic Unconstrained Binary Optimization (HUBO/QUBO) Problems, which are the standard input of classical and quantum combinatorial optimization techniques. We prove the validity of our approach by performing a numerical experiment based on data gathered from Twitter for a network of ten people, finding good agreement between results from a poll and those predicted by our model. In general terms, our method can also be understood as a trend detection algorithm, particularly useful in the contexts of sentiment analysis and identification of fake news.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Wang, R.; Hernani-Morales, C.; Martín-Guerrero, J. D.; Solano, E.; Albarrán-Arriagada, F.

Quantum Pattern Recognition in Photonic Circuits Journal Article

In: Quantum Science and Technology, vol. 7, no. 1, pp. 015010, 2022.

@article{Wang2022c,

title = {Quantum Pattern Recognition in Photonic Circuits},

author = {R. Wang and C. Hernani-Morales and J. D. Martín-Guerrero and E. Solano and F. Albarrán-Arriagada},

doi = {10.1088/2058-9565/ac3460},

year = {2022},

date = {2022-11-01},

journal = {Quantum Science and Technology},

volume = {7},

number = {1},

pages = {015010},

publisher = {IOP Publishing},

abstract = {This paper proposes a machine learning method to characterize photonic states via a simple optical circuit and data processing of photon number distributions, such as photonic patterns. The input states consist of two coherent states used as references and a two-mode unknown state to be studied. We successfully trained supervised learning algorithms that can predict the degree of entanglement in the two-mode state as well as perform the full tomography of one photonic mode, obtaining satisfactory values in the considered regression metrics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Barraza, N.; Pan, C. -Y.; Lamata, L.; Solano, E.; Albarrán-Arriagada, F.

Adaptive random quantum eigensolver Journal Article

In: Physical Review A, vol. 105, no. 5, pp. 052406, 2022.

@article{Barraza2022,

title = {Adaptive random quantum eigensolver},

author = {N. Barraza and C. -Y. Pan and L. Lamata and E. Solano and F. Albarrán-Arriagada},

doi = {10.1103/physreva.105.052406},

year = {2022},

date = {2022-05-01},

journal = {Physical Review A},

volume = {105},

number = {5},

pages = {052406},

publisher = {American Physical Society (APS)},

abstract = {We propose an adaptive random quantum algorithm to obtain an optimized eigensolver. Specifically, we introduce a general method to parametrize and optimize the probability density function of a random number generator, which is the core of stochastic algorithms. We follow a bioinspired evolutionary mutation method to introduce changes in the involved matrices. Our optimization is based on two figures of merit: learning speed and learning accuracy. This method provides high fidelities for the searched eigenvectors and faster convergence on the way to quantum advantage with current noisy intermediate-scaled quantum computers.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Casariego, M.; Cruzeiro, E. Z.; Gherardini, S.; Gonzalez-Raya, T.; André, R.; Frazão, G.; Catto, G.; Möttönen, M.; Datta, D.; Viisanen, K.; Govenius, J.; Prunnila, M.; Tuominen, K.; Reichert, M.; Renger, M.; Fedorov, K. G.; Deppe, F.; Vliet, H.; Matthews, A. J.; Fernández, Y.; Assouly, R.; Dassonneville, R.; Huard, B.; Sanz, M.; Omar, Y.

Propagating Quantum Microwaves: Towards Applications in Communication and Sensing Miscellaneous

2022.

@misc{Casariego2022,

title = {Propagating Quantum Microwaves: Towards Applications in Communication and Sensing},

author = {M. Casariego and E. Z. Cruzeiro and S. Gherardini and T. Gonzalez-Raya and R. André and G. Frazão and G. Catto and M. Möttönen and D. Datta and K. Viisanen and J. Govenius and M. Prunnila and K. Tuominen and M. Reichert and M. Renger and K. G. Fedorov and F. Deppe and H. Vliet and A. J. Matthews and Y. Fernández and R. Assouly and R. Dassonneville and B. Huard and M. Sanz and Y. Omar},

year = {2022},

date = {2022-05-01},

abstract = {The field of propagating quantum microwaves has started to receive considerable attention in the past few years. Motivated at first by the lack of an efficient microwave-to-optical platform that could solve the issue of secure communication between remote superconducting chips, current efforts are starting to reach other areas, from quantum communications to sensing. Here, we attempt at giving a state-of-the-art view of the two, pointing at some of the technical and theoretical challenges we need to address, and while providing some novel ideas and directions for future research. Hence, the goal of this paper is to provide a bigger picture, and -- we hope -- to inspire new ideas in quantum communications and sensing: from open-air microwave quantum key distribution to direct detection of dark matter, we expect that the recent efforts and results in quantum microwaves will soon attract a wider audience, not only in the academic community, but also in an industrial environment.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Yu, J.; Retamal, J. C.; Sanz, M.; Solano, E.; Albarrán-Arriagada, F.

Superconducting circuit architecture for digital-analog quantum computing Journal Article

In: EPJ Quantum Technology, vol. 9, no. 1, 2022.

@article{Yu2022,

title = {Superconducting circuit architecture for digital-analog quantum computing},

author = {J. Yu and J. C. Retamal and M. Sanz and E. Solano and F. Albarrán-Arriagada},

doi = {10.1140/epjqt/s40507-022-00129-y},

year = {2022},

date = {2022-04-01},

journal = {EPJ Quantum Technology},

volume = {9},

number = {1},

publisher = {Springer Science and Business Media LLC},

abstract = {We propose a superconducting circuit architecture suitable for digital-analog quantum computing (DAQC) based on an enhanced NISQ family of nearest-neighbor interactions. DAQC makes a smart use of digital steps (single qubit rotations) and analog blocks (parametrized multiqubit operations) to outperform digital quantum computing algorithms. Our design comprises a chain of superconducting charge qubits coupled by superconducting quantum interference devices (SQUIDs). Using magnetic flux control, we can activate/deactivate exchange interactions, double excitation/de-excitations, and others. As a paradigmatic example, we present an efficient simulation of an $elltimes h$ fermion lattice (with $2keywords = {},

pubstate = {published},

tppubtype = {article}

}

Coutinho, B. C.; Munro, W. J.; Nemoto, K.; Omar, Y.

Robustness of noisy quantum networks Journal Article

In: Communications Physics, vol. 5, no. 1, 2022.

@article{Coutinho2022,

title = {Robustness of noisy quantum networks},

author = {B. C. Coutinho and W. J. Munro and K. Nemoto and Y. Omar},

doi = {10.1038/s42005-022-00866-7},

year = {2022},

date = {2022-04-01},

journal = {Communications Physics},

volume = {5},

number = {1},

publisher = {Springer Science and Business Media LLC},

abstract = {Quantum networks allow us to harness networked quantum technologies and to develop a quantum internet. But how robust is a quantum network when its links and nodes start failing? We show that quantum complex networks based on typical noisy quantum-repeater nodes are prone to discontinuous phase transitions with respect to the random loss of operating links and nodes, abruptly compromising the connectivity of the network, and thus significantly limiting the reach of its operation. Furthermore, we determine the critical quantum-repeater efficiency necessary to avoid this catastrophic loss of connectivity as a function of the network topology, the network size, and the distribution of entanglement in the network. From all the network topologies tested, a scale-free network topology shows the best promise for a robust large-scale quantum internet.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Ai, M. -Z.; Ding, Y.; Ban, Y.; Martín-Guerrero, J. D.; Casanova, J.; Cui, J. -M.; Huang, Y. -F.; Chen, X.; Li, C. -F.; Guo, G. -C.

Experimentally realizing efficient quantum control with reinforcement learning Journal Article

In: Science China Physics, Mechanics &$mathsemicolon$ Astronomy, vol. 65, no. 5, 2022.

@article{Ai2022,

title = {Experimentally realizing efficient quantum control with reinforcement learning},

author = {M. -Z. Ai and Y. Ding and Y. Ban and J. D. Martín-Guerrero and J. Casanova and J. -M. Cui and Y. -F. Huang and X. Chen and C. -F. Li and G. -C. Guo},

doi = {10.1007/s11433-021-1841-2},

year = {2022},

date = {2022-03-01},

journal = {Science China Physics, Mechanics &$mathsemicolon$ Astronomy},

volume = {65},

number = {5},

publisher = {Springer Science and Business Media LLC},

abstract = {We experimentally investigate deep reinforcement learning (DRL) as an artificial intelligence approach to control a quantum system. We verify that DRL explores fast and robust digital quantum controls with operation time analytically hinted by shortcuts to adiabaticity. In particular, the protocol's robustness against both over-rotations and off-resonance errors can still be achieved simultaneously without any priori input. For the thorough comparison, we choose the task as single-qubit flipping, in which various analytical methods are well-developed as the benchmark, ensuring their feasibility in the quantum system as well. Consequently, a gate operation is demonstrated on a trapped 171Yb+ ion, significantly outperforming analytical pulses in the gate time and energy cost with hybrid robustness, as well as the fidelity after repetitive operations under time-varying stochastic errors. Our experiments reveal a framework of computer-inspired quantum control, which can be extended to other complicated tasks without loss of generality.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Ding, Y.; Martín-Guerrero, J. D.; Song, Y.; Magdalena-Benedito, R.; Chen, X.

Active learning for the optimal design of multinomial classification in physics Journal Article

In: Physical Review Research, vol. 4, no. 1, pp. 013213, 2022.

@article{Ding2022,

title = {Active learning for the optimal design of multinomial classification in physics},

author = {Y. Ding and J. D. Martín-Guerrero and Y. Song and R. Magdalena-Benedito and X. Chen},

doi = {10.1103/physrevresearch.4.013213},

year = {2022},

date = {2022-03-01},

journal = {Physical Review Research},

volume = {4},

number = {1},

pages = {013213},

publisher = {American Physical Society (APS)},

abstract = {Optimal design for model training is a critical topic in machine learning. Active learning aims at obtaining improved models by querying samples with maximum uncertainty according to the estimation model for artificially labeling; this has the additional advantage of achieving successful performances with a reduced number of labeled samples. We analyze its capability as an assistant for the design of experiments, extracting maximum information for learning with the minimal cost in fidelity loss, or reducing total operation costs of labeling in the laboratory. We present two typical applications as quantum information retrieval in qutrits and phase boundary prediction in many-body physics. For an equivalent multinomial classification problem, we achieve the correct rate of 99% with less than 2% of samples labeled. We reckon that active-learning-inspired physics experiments will remarkably save budget without loss of accuracy.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Fesquet, F.; Kronowetter, F.; Renger, M.; Chen, Q.; Honasoge, K.; Gargiulo, O.; Nojiri, Y.; Marx, A.; Deppe, F.; Gross, R.; Fedorov, K. G.

Perspectives of microwave quantum key distribution in open-air Miscellaneous

2022.

@misc{Fesquet2022,

title = {Perspectives of microwave quantum key distribution in open-air},

author = {F. Fesquet and F. Kronowetter and M. Renger and Q. Chen and K. Honasoge and O. Gargiulo and Y. Nojiri and A. Marx and F. Deppe and R. Gross and K. G. Fedorov},

url = {https://arxiv.org/abs/2203.05530},

year = {2022},

date = {2022-03-01},

abstract = {One of the cornerstones of quantum communication is the unconditionally secure distribution of classical keys between remote parties. This key feature of quantum technology is based on the quantum properties of propagating electromagnetic waves, such as entanglement, or the no-cloning theorem. However, these quantum resources are known to be susceptible to noise and losses, which are omnipresent in open-air communication scenarios. In this work, we theoretically investigate the perspectives of continuous-variable open-air quantum key distribution at microwave frequencies. In particular, we present a model describing the coupling of propagating microwaves with a noisy environment. Using a protocol based on displaced squeezed states, we demonstrate that continuous-variable quantum key distribution with propagating microwaves can be unconditionally secure at room temperature up to distances of around 200 meters. Moreover, we show that microwaves can potentially outperform conventional quantum key distribution at telecom wavelength at imperfect weather conditions.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Hyyppä, E.; Kundu, S.; Chan, C. F.; Gunyhó, A.; Hotari, J.; Kiuru, O.; Landra, A.; Liu, W.; Marxer, F.; Mäkinen, A.; Orgiazzi, J. -L.; Palma, M.; Savytskyi, M.; Tosto, F.; Tuorila, J.; Vadimov, V.; Li, T.; Ockeloen-Korppi, C.; Heinsoo, J.; Tan, K. Y.; Hassel, J.; Möttönen, M.

Unimon qubit Miscellaneous

2022.

@misc{Hyyppae2022,

title = {Unimon qubit},

author = {E. Hyyppä and S. Kundu and C. F. Chan and A. Gunyhó and J. Hotari and O. Kiuru and A. Landra and W. Liu and F. Marxer and A. Mäkinen and J. -L. Orgiazzi and M. Palma and M. Savytskyi and F. Tosto and J. Tuorila and V. Vadimov and T. Li and C. Ockeloen-Korppi and J. Heinsoo and K. Y. Tan and J. Hassel and M. Möttönen},

url = {https://arxiv.org/abs/2203.05896},

year = {2022},

date = {2022-03-01},

abstract = {Superconducting qubits are one of the most promising candidates to implement quantum computers. The superiority of superconducting quantum computers over any classical device in simulating random but well-determined quantum circuits has already been shown in two independent experiments and important steps have been taken in quantum error correction. However, the currently wide-spread qubit designs do not yet provide high enough performance to enable practical applications or efficient scaling of logical qubits owing to one or several following issues: sensitivity to charge or flux noise leading to decoherence, too weak non-linearity preventing fast operations, undesirably dense excitation spectrum, or complicated design vulnerable to parasitic capacitance. Here, we introduce and demonstrate a superconducting-qubit type, the unimon, which combines the desired properties of high non-linearity, full insensitivity to dc charge noise, insensitivity to flux noise, and a simple structure consisting only of a single Josephson junction in a resonator. We measure the qubit frequency, $ømega_01/(2pi)$, and anharmonicity $alpha$ over the full dc-flux range and observe, in agreement with our quantum models, that the qubit anharmonicity is greatly enhanced at the optimal operation point, yielding, for example, 99.9% and 99.8% fidelity for 13-ns single-qubit gates on two qubits with $(ømega_01,alpha)=(4.49~mathrmGHz, 434~mathrm MHz)times 2pi$ and $(3.55~mathrmGHz, 744~mathrm MHz)times 2pi$, respectively. The energy relaxation time $T_1łesssim 10~mumathrms$ is stable for hours and seems to be limited by dielectric losses. Thus, future improvements of the design, materials, and gate time may promote the unimon to break the 99.99% fidelity target for efficient quantum error correction and possible quantum advantage with noisy systems.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Gonzalez-Raya, T.; Casariego, M.; Fesquet, F.; Renger, M.; Salari, V.; Möttönen, M.; Omar, Y.; Deppe, F.; Fedorov, K. G.; Sanz, M.

Open-Air Microwave Entanglement Distribution for Quantum Teleportation Miscellaneous

2022.

@misc{GonzalezRaya2022,

title = {Open-Air Microwave Entanglement Distribution for Quantum Teleportation},

author = {T. Gonzalez-Raya and M. Casariego and F. Fesquet and M. Renger and V. Salari and M. Möttönen and Y. Omar and F. Deppe and K. G. Fedorov and M. Sanz},

url = {https://arxiv.org/abs/2203.07295},

year = {2022},

date = {2022-03-01},

abstract = {Microwave technology plays a central role in current wireless communications, standing among them mobile communication and local area networks (LANs). The microwave range shows relevant advantages with respect to other frequencies in open-air transmission, such as low absorption losses and low energy consumption, and it is additionally the natural working frequency in superconducting quantum technologies. Entanglement distribution between separate parties is at the core of secure quantum communications. Therefore, understanding its limitations in realistic open-air settings, specially in the rather unexplored microwave regime, is crucial for transforming microwave quantum communications into a mainstream technology. Here, we investigate the feasibility of an open-air entanglement distribution scheme with microwave two-mode squeezed states. First, we study the reach of direct entanglement transmission in open-air, obtaining a maximum distance of approximately 500 meters in a realistic setting with state-of-the-art experimental parameters. Afterwards, we adapt entanglement distillation and entanglement swapping protocols to microwave technology in order to reduce environmental entanglement degradation. While entanglement distillation helps to increase quantum correlations in the short-distance low-squeezing regime by up to $46%$, entanglement swapping increases the reach by $14%$. Then, we compute the fidelity of a continuous-variable quantum teleportation protocol using open-air-distributed entanglement as a resource. Finally, we adapt the machinery to explore the limitations of quantum communication between satellites, where the thermal noise impact is substantially reduced and diffraction losses are dominant.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Pauwels, J.; Pironio, S.; Cruzeiro, E. Z.; Tavakoli, A.

Adaptive advantage in entanglement-assisted communications Miscellaneous

2022.

@misc{Pauwels2022,

title = {Adaptive advantage in entanglement-assisted communications},

author = {J. Pauwels and S. Pironio and E. Z. Cruzeiro and A. Tavakoli},

url = {https://arxiv.org/abs/2203.05372},

year = {2022},

date = {2022-03-01},

abstract = {Entanglement is known to boost the efficiency of classical communication. In distributed computation, for instance, exploiting entanglement can reduce the number of communicated bits or increase the probability to obtain a correct answer. Entanglement-assisted classical communication protocols usually consist of two successive rounds: first a Bell test round, in which the parties measure their local shares of the entangled state, and then a communication round, where they exchange classical messages. Here, we go beyond this standard approach and investigate adaptive uses of entanglement: we allow the receiver to wait for the arrival of the sender's message before measuring his share of the entangled state. We first show that such adaptive protocols improve the success probability in Random Access Codes. Second, we show that once adaptive measurements are used, an entanglement-assisted bit becomes a strictly stronger resource than a qubit in prepare-and-measure scenarios. We briefly discuss extension of these ideas to scenarios involving quantum communication and identify resource inequalities.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Buffoni, L.; Gherardini, S.; Cruzeiro, E. Z.; Omar, Y.

Third law of thermodynamics and the scaling of quantum computers Miscellaneous

2022.

@misc{Buffoni2022,

title = {Third law of thermodynamics and the scaling of quantum computers},

author = {L. Buffoni and S. Gherardini and E. Z. Cruzeiro and Y. Omar},

url = {https://arxiv.org/abs/2203.09545},

year = {2022},

date = {2022-03-01},

abstract = {The third law of thermodynamics, also known as the Nernst unattainability principle, puts a fundamental bound on how close a system, whether classical or quantum, can be cooled to a temperature near to absolute zero. On the other hand, a fundamental assumption of quantum computing is to start each computation from a register of qubits initialized in a pure state, i.e. at zero temperature. These conflicting aspects, at the interface between quantum computing and thermodynamics, are often overlooked or, at best, addressed only at a single-qubit level. In this work, we argue how the existence of a small, but finite, effective temperature, which makes the initial state a mixed state, poses a real challenge to the fidelity constraints required for the scaling of quantum computers. Our theoretical results, carried out for a generic quantum circuit with $N$-qubit input states, are validated by experiments performed on a real quantum processor.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Ibarrondo, R.; Gatti, G.; Sanz, M.

Quantum Genetic Algorithm with Individuals in Multiple Registers Miscellaneous

2022.

@misc{Ibarrondo2022,

title = {Quantum Genetic Algorithm with Individuals in Multiple Registers},

author = {R. Ibarrondo and G. Gatti and M. Sanz},

url = {https://arxiv.org/abs/2203.15039},

year = {2022},

date = {2022-03-01},

abstract = {Genetic algorithms are heuristic optimization techniques inspired by Darwinian evolution, which are characterized by successfully finding robust solutions for optimization problems. Here, we propose a subroutine-based quantum genetic algorithm with individuals codified in independent registers. This distinctive codification allows our proposal to depict all the fundamental elements characterizing genetic algorithms, i.e. population-based search with selection of many individuals, crossover, and mutation. Our subroutine-based construction permits us to consider several variants of the algorithm. For instance, we firstly analyze the performance of two different quantum cloning machines, a key component of the crossover subroutine. Indeed, we study two paradigmatic examples, namely, the biomimetic cloning of quantum observables and the Buv zek-Hillery universal quantum cloning machine, observing a faster average convergence of the former, but better final populations of the latter. Additionally, we analyzed the effect of introducing a mutation subroutine, concluding a minor impact on the average performance. Furthermore, we introduce a quantum channel analysis to prove the exponential convergence of our algorithm and even predict its convergence-ratio. This tool could be extended to formally prove results on the convergence of general non-unitary iteration-based algorithms.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Reichert, M.; Candia, R. D.; Win, M. Z.; Sanz, M.

Quantum-Enhanced Doppler Radar/Lidar Miscellaneous

2022.

@misc{Reichert2022,

title = {Quantum-Enhanced Doppler Radar/Lidar},

author = {M. Reichert and R. D. Candia and M. Z. Win and M. Sanz},

url = {https://arxiv.org/abs/2203.16424},

doi = {10.48550/arXiv.2203.16424},

year = {2022},

date = {2022-03-01},

abstract = {We propose a quantum-enhanced protocol to estimate the radial velocity $v$ of a moving target, using a frequency-entangled squeezed state composed of a signal and an idler beam as a probe state. The signal beam illuminates the moving object and it is reflected with its frequency shifted due to the Doppler effect. Then, a joint measurement between signal and idler beams is performed to estimate the velocity of the object. We aim at benchmarking this protocol against the classical one, which comprises a coherent state with the same energy illuminating the object. Indeed, employing squeezing and frequency entanglement as quantum resources provides to a precision enhancement in the estimation of the velocity of the object. We identify three distinct parameter regimes. First, the frequency entanglement-dominant regime, where the advantage is proportional to the degree of frequency entanglement and mostly insensitive to the photon number. Second, the squeezing-dominant regime, with a quantum advantage that is higher than the standard quantum limit. Third, the mixed regime, where both squeezing and frequency entanglement are comparable and the proposed quantum protocol attains the Heisenberg limit. We show that an optimal measurement to achieve these results is frequency-resolved photon-number counting. Losses in the signal beam are considered for the high frequency entanglement regime. The protocol shows resilience, outperforming the classical protocol for all channel transmissivities given a large enough frequency entanglement.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Chandarana, P.; Hegade, N. N.; Paul, Koushik; Albarrán-Arriagada, F.; Solano, Enrique; Campo, A.; Chen, Xi

Digitized-counterdiabatic quantum approximate optimization algorithm Journal Article

In: Physical Review Research, vol. 4, no. 1, pp. 013141, 2022.

@article{Chandarana2022,

title = {Digitized-counterdiabatic quantum approximate optimization algorithm},

author = {P. Chandarana and N. N. Hegade and Koushik Paul and F. Albarrán-Arriagada and Enrique Solano and A. Campo and Xi Chen},

doi = {10.1103/physrevresearch.4.013141},

year = {2022},

date = {2022-02-01},

journal = {Physical Review Research},

volume = {4},

number = {1},

pages = {013141},

publisher = {American Physical Society (APS)},

abstract = {The quantum approximate optimization algorithm (QAOA) has proved to be an effective classical-quantum algorithm serving multiple purposes, from solving combinatorial optimization problems to finding the ground state of many-body quantum systems. Since the QAOA is an Ansatz-dependent algorithm, there is always a need to design Ansaetze for better optimization. To this end, we propose a digitized version of the QAOA enhanced via the use of shortcuts to adiabaticity. Specifically, we use a counterdiabatic (CD) driving term to design a better Ansatz, along with the Hamiltonian and mixing terms, enhancing the global performance. We apply our digitized-CD QAOA to Ising models, classical optimization problems, and the P-spin model, demonstrating that it outperforms the standard QAOA in all cases we study.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Chen, Q. -M.; Kronowetter, F.; Fesquet, F.; Honasoge, K. E.; Nojiri, Y.; Renger, M.; Fedorov, K. G.; Marx, A.; Deppe, F.; Gross, R.

Tuning and amplifying the interactions in superconducting quantum circuits with subradiant qubits Journal Article

In: Physical Review A, vol. 105, no. 1, pp. 012405, 2022.

@article{Chen2022,

title = {Tuning and amplifying the interactions in superconducting quantum circuits with subradiant qubits},

author = {Q. -M. Chen and F. Kronowetter and F. Fesquet and K. E. Honasoge and Y. Nojiri and M. Renger and K. G. Fedorov and A. Marx and F. Deppe and R. Gross},

doi = {10.1103/physreva.105.012405},

year = {2022},

date = {2022-01-01},

journal = {Physical Review A},

volume = {105},

number = {1},

pages = {012405},

publisher = {American Physical Society (APS)},

abstract = {We propose a tunable coupler consisting of N fixed-frequency qubits, which can tune and even amplify the effective interaction between two superconducting quantum circuits. The tuning range of the interaction is proportional to N, with a minimum value of zero and a maximum that can exceed the physical coupling rates between the coupler and the circuits. The effective coupling rate is determined by the collective magnetic quantum number of the qubit ensemble, which takes only discrete values and is free from collective decay and decoherence. Using single-photon pi-pulses, the coupling rate can be switched between arbitrary choices of the initial and final values within the dynamic range in a single step without going through intermediate values. A cascade of the couplers for amplifying small interactions or weak signals is also discussed. These results should not only stimulate interest in exploring the collective effects in quantum information processing, but also enable development of applications in tuning and amplifying the interactions in a general cavity-QED system.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Cárdenas-López, F. A.; Chen, X.

Shortcuts to Adiabaticity for Fast Qubit Readout in Circuit Quantum Electrodynamics Miscellaneous

2022.

@misc{CardenasLopez2022,

title = {Shortcuts to Adiabaticity for Fast Qubit Readout in Circuit Quantum Electrodynamics},

author = {F. A. Cárdenas-López and X. Chen},

url = {https://arxiv.org/abs/2201.06007},

year = {2022},

date = {2022-01-01},

abstract = {We propose how to engineer the longitudinal coupling to accelerate the measurement of a qubit longitudinally coupled to a cavity, motivated by the concept of shortcuts to adiabaticity. Different modulations are inversely designed from two methods of inverse engineering and counter-diabatic driving, for achieving larger values of the signal-to-noise ratio (SNR) at nanosecond scale. By comparison, we demonstrate that our protocols outperform the usual periodic modulations on the pointer state separation and SNR. Finally, we show a possible implementation considering state-of-the-art circuit quantum electrodynamics architecture, estimating the minimal time allowed for the measurement process.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Moutinho, J. P.; Coutinho, B.; Buffoni, L.

Network-based link prediction of scientific concepts -- a Science4Cast competition entry Miscellaneous

2022.

@misc{Moutinho2022,

title = {Network-based link prediction of scientific concepts -- a Science4Cast competition entry},

author = {J. P. Moutinho and B. Coutinho and L. Buffoni},

year = {2022},

date = {2022-01-01},

journal = {2021 IEEE International Conference on Big Data (Big Data) (pp. 5815-5819). IEEE},

abstract = {We report on a model built to predict links in a complex network of scientific concepts, in the context of the Science4Cast 2021 competition. We show that the network heavily favours linking nodes of high degree, indicating that new scientific connections are primarily made between popular concepts, which constitutes the main feature of our model. Besides this notion of popularity, we use a measure of similarity between nodes quantified by a normalized count of their common neighbours to improve the model. Finally, we show that the model can be further improved by considering a time-weighted adjacency matrix with both older and newer links having higher impact in the predictions, representing rooted concepts and state of the art research, respectively.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

### 2021

Kumar, S.; Cárdenas-López, F. A.; Hegade, N. N.; Chen, X.; Albarrán-Arriagada, F.; Solano, E.; Barrios, G. A.

Entangled quantum memristors Journal Article

In: Physical Review A, vol. 104, no. 6, pp. 062605, 2021.

@article{Kumar2021,

title = {Entangled quantum memristors},

author = {S. Kumar and F. A. Cárdenas-López and N. N. Hegade and X. Chen and F. Albarrán-Arriagada and E. Solano and G. A. Barrios},

doi = {10.1103/physreva.104.062605},

year = {2021},

date = {2021-12-01},

journal = {Physical Review A},

volume = {104},

number = {6},

pages = {062605},

publisher = {American Physical Society (APS)},

abstract = {We propose the interaction of two quantum memristors via capacitive and inductive coupling in feasible superconducting circuit architectures. In this composed system the input gets correlated in time, which changes the dynamic response of each quantum memristor in terms of its pinched hysteresis curve and nontrivial entanglement. In this sense, the concurrence and memristive dynamics follow an inverse behavior, showing maximal values of entanglement when the hysteresis curve is minimal and vice versa. Moreover, the direction followed in time by the hysteresis curve is reversed whenever the quantum memristor entanglement is maximal. The study of composed quantum memristors paves the way for developing neuromorphic quantum computers and native quantum neural networks, on the path towards quantum advantage with current noisy intermediate-scale quantum era technologies.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Fedorov, K. G.; Renger, M.; Pogorzalek, S.; Candia, R. D.; Chen, Q.; Nojiri, Y.; Inomata, K.; Nakamura, Y.; Partanen, M.; Marx, A.; Gross, R.; Deppe, F.

Experimental quantum teleportation of propagating microwaves Journal Article

In: Science Advances, vol. 7, no. 52, 2021.

@article{Fedorov2021b,

title = {Experimental quantum teleportation of propagating microwaves},

author = {K. G. Fedorov and M. Renger and S. Pogorzalek and R. D. Candia and Q. Chen and Y. Nojiri and K. Inomata and Y. Nakamura and M. Partanen and A. Marx and R. Gross and F. Deppe},

doi = {10.1126/sciadv.abk0891},

year = {2021},

date = {2021-12-01},

journal = {Science Advances},

volume = {7},

number = {52},

publisher = {American Association for the Advancement of Science (AAAS)},

abstract = {The field of quantum communication promises to provide efficient and unconditionally secure ways to exchange information, particularly, in the form of quantum states. Meanwhile, recent breakthroughs in quantum computation with superconducting circuits trigger a demand for quantum communication channels between spatially separated superconducting processors operating at microwave frequencies. In pursuit of this goal, we demonstrate the unconditional quantum teleportation of propagating coherent microwave states by exploiting two-mode squeezing and analog feedforward over a macroscopic distance of d = 0.42 m. We achieve a teleportation fidelity of F = 0.689 pm 0.004, exceeding the asymptotic no-cloning threshold. Thus, the quantum nature of the teleported states is preserved, opening the avenue toward unconditional security in microwave quantum communication.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Ding, Y.; Chen, X.; Magdalena-Benedicto, R.; Martín-Guerrero, J. D.

Quantum Stream Learning Miscellaneous

2021.

@misc{Ding2021f,

title = {Quantum Stream Learning},

author = {Y. Ding and X. Chen and R. Magdalena-Benedicto and J. D. Martín-Guerrero},

url = {https://arxiv.org/abs/2112.06628},

year = {2021},

date = {2021-12-01},

abstract = {The exotic nature of quantum mechanics makes machine learning (ML) be different in the quantum realm compared to classical applications. ML can be used for knowledge discovery using information continuously extracted from a quantum system in a broad range of tasks. The model receives streaming quantum information for learning and decision-making, resulting in instant feedback on the quantum system. As a stream learning approach, we present a deep reinforcement learning on streaming data from a continuously measured qubit at the presence of detuning, dephasing, and relaxation. We also investigate how the agent adapts to another quantum noise pattern by transfer learning. Stream learning provides a better understanding of closed-loop quantum control, which may pave the way for advanced quantum technologies.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Moutinho, J. P.; Melo, A.; Coutinho, B.; Kovács, I. A.; Omar, Y.

Quantum Link Prediction in Complex Networks Miscellaneous

2021.

@misc{Moutinho2021,

title = {Quantum Link Prediction in Complex Networks},

author = {J. P. Moutinho and A. Melo and B. Coutinho and I. A. Kovács and Y. Omar},

url = {https://arxiv.org/abs/2112.04768},

year = {2021},

date = {2021-12-01},

abstract = {Predicting new links in physical, biological, social, or technological networks has a significant scientific and societal impact. Network-based link prediction methods utilize topological patterns in a network to infer new or unobserved links. Here, we propose a quantum algorithm for link prediction, QLP, which uses quantum walks to infer unknown links based on even and odd length paths. By sampling new links from quantum measurements, QLP avoids the need to explicitly calculate all pairwise scores in the network. We study the complexity of QLP and discuss in which cases one may achieve a polynomial speedup over classical link prediction methods. Furthermore, tests with real-world datasets show that QLP is at least as precise as state-of-the-art classical link prediction methods, both in cross-validation tests and in the prediction of experimentally verified protein-protein interactions.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Ma, D.; Jia, C.; Solano, E.; Céleri, L. C.

Analogue gravitational lensing in optical Bose-Einstein condensates Miscellaneous

2021.

@misc{Ma2021b,

title = {Analogue gravitational lensing in optical Bose-Einstein condensates},

author = {D. Ma and C. Jia and E. Solano and L. C. Céleri},

url = {https://arxiv.org/abs/2112.06235},

year = {2021},

date = {2021-12-01},

abstract = {We consider acoustic propagation of phonons in the presence of a non-rotating vortex with radial flow in a Bose-Einstein condensate (BEC) of photons. Since the vortex can be used to simulate a static acoustic black hole, the phonon would experience a considerable spacetime curvature at appreciable distance from the vortex core. The trajectory of the phonons is bended after passing by the vortex, which can be used as a simulation of gravitational lensing for phonons in a photonic BEC.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Koch, J.; Hunanyan, G.; Ockenfels, T.; Rico, E.; Solano, E.; Weitz, M.

Quantum Rabi dynamics of trapped atoms far in the deep strong coupling regime Miscellaneous

2021.

@misc{Koch2021,

title = {Quantum Rabi dynamics of trapped atoms far in the deep strong coupling regime},

author = {J. Koch and G. Hunanyan and T. Ockenfels and E. Rico and E. Solano and M. Weitz},

url = {https://arxiv.org/abs/2112.12488},

year = {2021},

date = {2021-12-01},

abstract = {The coupling of a two-level system with an electromagnetic field, whose fully quantized field version is known as the quantum Rabi model is among the central topics of quantum physics and recent quantum information technologies. When the coupling strength becomes stronger than the decoherence rate the so-called strong coupling regime is reached, with mixed states of the two-level system and the field mode becoming relevant. Further, when the coupling strength reaches the field mode frequency the deep strong coupling regime is approached and excitations can be created out of the vacuum, see also recent works with effective spin-field, trapped ion, and superconducting qubit-oscillator implementations. Here we demonstrate a novel approach for the realization of a periodic variant of the quantum Rabi model using two coupled quantized mechanical modes of cold atoms in optical potentials, which has allowed us to reach a Rabi coupling strength of 6.5 times the field mode frequency, i.e., far in the deep strong coupling regime. For the first time, the coupling term dominates over all other energy scales. Field mode creation and annihilation upon e.g., de-excitation of the two-level system here approach equal magnitudes, and we observe the atomic dynamics in this novel experimental regime, revealing a subcycle timescale raise in bosonic field mode excitations, in good agreement with theoretical predictions. In a measurement recorded in the basis of the coupling term of the quantum Rabi Hamiltonian, the observed dynamics freezes for small frequency splittings of the two-level system, but revives for larger splittings. Our concept demonstrates a route to realize quantum-engineering applications in yet unexplored parameter regimes.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Ibarrondo, R.; Sanz, M.; Orus, R.

Forecasting Election Polls with Spin Systems Journal Article

In: SN Computer Science, vol. 3, no. 1, 2021.

@article{Ibarrondo2021,

title = {Forecasting Election Polls with Spin Systems},

author = {R. Ibarrondo and M. Sanz and R. Orus},

doi = {10.1007/s42979-021-00942-9},

year = {2021},

date = {2021-11-01},

journal = {SN Computer Science},

volume = {3},

number = {1},

publisher = {Springer Science and Business Media LLC},

abstract = {We show that the problem of political forecasting, i.e, predicting the result of elections and referendums, can be mapped to finding the ground-state configuration of a classical spin system. Depending on the required prediction, this spin system can be a combination of XY, Ising and vector Potts models, always with two-spin interactions, magnetic fields, and on arbitrary graphs. By reduction to the Ising model, our result shows that political forecasting is formally an NP-Hard problem. Moreover, we show that the ground state search can be recasted as Higher order and Quadratic Unconstrained Binary Optimization (HUBO/QUBO) Problems, which are the standard input of classical and quantum combinatorial optimization techniques. We prove the validity of our approach by performing a numerical experiment based on data gathered from Twitter for a network of ten people, finding good agreement between results from a poll and those predicted by our model. In general terms, our method can also be understood as a trend detection algorithm, particularly useful in the contexts of sentiment analysis and identification of fake news.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Renger, M.; Pogorzalek, S.; Chen, Q.; Nojiri, Y.; Inomata, K.; Nakamura, Y.; Partanen, M.; Marx, A.; Gross, R.; Deppe, F.; Fedorov, K. G.

Beyond the standard quantum limit for parametric amplification of broadband signals Journal Article

In: npj Quantum Information, vol. 7, no. 1, pp. 160, 2021.

@article{Renger2021,

title = {Beyond the standard quantum limit for parametric amplification of broadband signals},

author = {M. Renger and S. Pogorzalek and Q. Chen and Y. Nojiri and K. Inomata and Y. Nakamura and M. Partanen and A. Marx and R. Gross and F. Deppe and K. G. Fedorov},

doi = {10.1038/s41534-021-00495-y},

year = {2021},

date = {2021-11-01},

journal = {npj Quantum Information},

volume = {7},

number = {1},

pages = {160},

publisher = {Springer Science and Business Media LLC},

abstract = {The low-noise amplification of weak microwave signals is crucial for countless protocols in quantum information processing. Quantum mechanics sets an ultimate lower limit of half a photon to the added input noise for phase-preserving amplification of narrowband signals, also known as the standard quantum limit (SQL). This limit, which is equivalent to a maximum quantum efficiency of 0.5, can be overcome by employing nondegenerate parametric amplification of broadband signals. We show that, in principle, a maximum quantum efficiency of unity can be reached. Experimentally, we find a quantum efficiency of 0.69 pm 0.02, well beyond the SQL, by employing a flux-driven Josephson parametric amplifier and broadband thermal signals. We expect that our results allow for fundamental improvements in the detection of ultraweak microwave signals.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Wang, R.; Hernani-Morales, C.; Martín-Guerrero, J. D.; Solano, E.; Albarrán-Arriagada, F.

Quantum Pattern Recognition in Photonic Circuits Journal Article

In: Quantum Science and Technology, vol. 7, no. 1, pp. 015010, 2021.

@article{Wang2021k,

title = {Quantum Pattern Recognition in Photonic Circuits},

author = {R. Wang and C. Hernani-Morales and J. D. Martín-Guerrero and E. Solano and F. Albarrán-Arriagada},

doi = {10.1088/2058-9565/ac3460},

year = {2021},

date = {2021-11-01},

journal = {Quantum Science and Technology},

volume = {7},

number = {1},

pages = {015010},

publisher = {IOP Publishing},

abstract = {This paper proposes a machine learning method to characterize photonic states via a simple optical circuit and data processing of photon number distributions, such as photonic patterns. The input states consist of two coherent states used as references and a two-mode unknown state to be studied. We successfully trained supervised learning algorithms that can predict the degree of entanglement in the two-mode state as well as perform the full tomography of one photonic mode, obtaining satisfactory values in the considered regression metrics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Hegade, N. N.; Paul, K.; Albarrán-Arriagada, F.; Chen, X.; Solano, E.

Digitized adiabatic quantum factorization Journal Article

In: Physical Review A, vol. 104, no. 5, pp. l050403, 2021.

@article{Hegade2021a,

title = {Digitized adiabatic quantum factorization},

author = {N. N. Hegade and K. Paul and F. Albarrán-Arriagada and X. Chen and E. Solano},

doi = {10.1103/physreva.104.l050403},

year = {2021},

date = {2021-11-01},

journal = {Physical Review A},

volume = {104},

number = {5},

pages = {l050403},

publisher = {American Physical Society (APS)},

abstract = {Quantum integer factorization is a potential quantum computing solution that may revolutionize cryptography. Nevertheless, a scalable and efficient quantum algorithm for noisy intermediate-scale quantum computers looks far-fetched. We propose an alternative factorization method, within the digitized-adiabatic quantum computing paradigm, by digitizing an adiabatic quantum factorization algorithm enhanced by shortcuts to adiabaticity techniques. We find that this fast factorization algorithm is suitable for available gate-based quantum computers. We test our quantum algorithm in an IBM quantum computer with up to six qubits, surpassing the performance of the more commonly used factorization algorithms on the long way towards quantum advantage.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Perelshtein, M.; Petrovnin, K.; Vesterinen, V.; Raja, S. H.; Lilja, I.; Will, M.; Savin, A.; Simbierowicz, S.; Jabdaraghi, R.; Lehtinen, J.; Grönberg, L.; Hassel, J.; Prunnila, M.; Govenius, J.; Paraoanu, S.; Hakonen, P.

Broadband continuous variable entanglement generation using Kerr-free Josephson metamaterial Miscellaneous

2021.

@misc{Perelshtein2021,

title = {Broadband continuous variable entanglement generation using Kerr-free Josephson metamaterial},

author = {M. Perelshtein and K. Petrovnin and V. Vesterinen and S. H. Raja and I. Lilja and M. Will and A. Savin and S. Simbierowicz and R. Jabdaraghi and J. Lehtinen and L. Grönberg and J. Hassel and M. Prunnila and J. Govenius and S. Paraoanu and P. Hakonen},

url = {https://arxiv.org/abs/2111.06145},

year = {2021},

date = {2021-11-01},

abstract = {Entangled microwave photons form a fundamental resource for quantum information processing and sensing with continuous variables. We use a low-loss Josephson metamaterial comprising superconducting non-linear asymmetric inductive elements to generate frequency (colour) entangled photons from vacuum fluctuations at a rate of 11 mega entangled bits per second with a potential rate above gigabit per second. The device is operated as a traveling wave parametric amplifier under Kerr-relieving biasing conditions. Furthermore, we realize the first successfully demonstration of single-mode squeezing in such devices - $2.4pm0.7$ dB below the zero-point level at half of modulation frequency.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Marin-Sanchez, G.; Gonzalez-Conde, J.; Sanz, M.

Quantum algorithms for approximate function loading Journal Article

In: 2021.

@article{MarinSanchez2021,

title = {Quantum algorithms for approximate function loading},

author = {G. Marin-Sanchez and J. Gonzalez-Conde and M. Sanz},

url = {https://arxiv.org/abs/2111.07933},

year = {2021},

date = {2021-11-01},

abstract = {Loading classical data into quantum computers represents an essential stage in many relevant quantum algorithms, especially in the field of quantum machine learning. Therefore, the inefficiency of this loading process means a major bottleneck for the application of these algorithms. Here, we introduce two approximate quantum-state preparation methods inspired by the Grover-Rudolph algorithm, which partially solve the problem of loading real functions. Indeed, by allowing for an infidelity $epsilon$ and under certain smoothness conditions, we prove that the complexity of Grover-Rudolph algorithm can be reduced from $mathcalO(2^n)$ to $mathcalO(2^k_0(epsilon))$, with $n$ the number of qubits and $k_0(epsilon)$ asymptotically independent of $n$. This leads to a dramatic reduction in the number of required two-qubit gates. Aroused by this result, we also propose a variational algorithm capable of loading functions beyond the aforementioned smoothness conditions. Our variational ansatz is explicitly tailored to the landscape of the function, leading to a quasi-optimized number of hyperparameters. This allows us to achieve high fidelity in the loaded state with high speed convergence for the studied examples.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Munuera-Javaloy, C.; Puebla, R.; D'Anjou, B.; Plenio, M. B.; Casanova, J.

Detection of Molecular Transitions with Nitrogen-Vacancy Centers and Electron-Spin Labels Miscellaneous

2021.

@misc{MunueraJavaloy2021a,

title = {Detection of Molecular Transitions with Nitrogen-Vacancy Centers and Electron-Spin Labels},

author = {C. Munuera-Javaloy and R. Puebla and B. D'Anjou and M. B. Plenio and J. Casanova},

url = {https://arxiv.org/abs/2110.14255},

year = {2021},

date = {2021-10-01},

abstract = {We present a protocol that detects molecular conformational changes with two nitroxide electron-spin labels and a nitrogen-vacancy (NV) center in diamond. More specifically, we demonstrate that the NV can detect energy shifts induced by the coupling between electron-spin labels. The protocol relies on the judicious application of microwave and radiofrequency pulses in a range of parameters that ensures stable nitroxide resonances. Furthermore, we demonstrate that our scheme is optimized by using nitroxides with distinct nitrogen isotopes. We use detailed numerical simulations and Bayesian inference techniques to demonstrate that our method enables the detection of conformational changes under realistic conditions including strong NV dephasing rates as a consequence of the diamond surface proximity and nitroxide thermalization mechanisms. Finally, we show that random molecular tumbling can be exploited to extract the inter-label distance.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Yan, Y.; Shi, C.; Kinos, A.; Syed, H.; Horvath, S.; Walther, A.; Rippe, L.; Chen, X.; Kröll, S.

Experimental implementation of precisely tailored light-matter interaction via inverse engineering Journal Article

In: npj Quantum Information, vol. 7, no. 1, pp. 138, 2021.

@article{Yan2021,

title = {Experimental implementation of precisely tailored light-matter interaction via inverse engineering},

author = {Y. Yan and C. Shi and A. Kinos and H. Syed and S. Horvath and A. Walther and L. Rippe and X. Chen and S. Kröll},

doi = {10.1038/s41534-021-00473-4},

year = {2021},

date = {2021-09-01},

journal = {npj Quantum Information},

volume = {7},

number = {1},

pages = {138},

publisher = {Springer Science and Business Media LLC},

abstract = {Accurate and efficient quantum control in the presence of constraints and decoherence is a requirement and a challenge in quantum information processing. Shortcuts to adiabaticity, originally proposed to speed up the slow adiabatic process, have nowadays become versatile toolboxes for preparing states or controlling the quantum dynamics. Unique shortcut designs are required for each quantum system with intrinsic physical constraints, imperfections, and noise. Here, we implement fast and robust control for the state preparation and state engineering in a rare-earth ions system. Specifically, the interacting pulses are inversely engineered and further optimized with respect to inhomogeneities of the ensemble and the unwanted interaction with other qubits. We demonstrate that our protocols surpass the conventional adiabatic schemes, by reducing the decoherence from the excited-state decay and inhomogeneous broadening. The results presented here are applicable to other noisy intermediate-scale quantum systems.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Hung, J. S.; Busnaina, J.; Chang, C. S.; Vadiraj, A.; Nsanzineza, I.; Solano, E.; Alaeian, H.; Rico, E.; Wilson, C.

Quantum Simulation of the Bosonic Creutz Ladder with a Parametric Cavity Journal Article

In: Physical Review Letters, vol. 127, no. 10, pp. 100503, 2021.

@article{Hung2021,

title = {Quantum Simulation of the Bosonic Creutz Ladder with a Parametric Cavity},

author = {J. S. Hung and J. Busnaina and C. S. Chang and A. Vadiraj and I. Nsanzineza and E. Solano and H. Alaeian and E. Rico and C. Wilson},

doi = {10.1103/physrevlett.127.100503},

year = {2021},

date = {2021-09-01},

journal = {Physical Review Letters},

volume = {127},

number = {10},

pages = {100503},

publisher = {American Physical Society (APS)},

abstract = {There has been a growing interest in realizing quantum simulators for physical systems where perturbative methods are ineffective. The scalability and flexibility of circuit quantum electrodynamics make it a promising platform for implementing various types of simulators, including lattice models of strongly coupled field theories. Here, we use a multimode superconducting parametric cavity as a hardware-efficient analog quantum simulator, realizing a lattice in synthetic dimensions with complex hopping interactions. The coupling graph, i.e., the realized model, can be programmed in situ. The complex-valued hopping interaction further allows us to simulate, for instance, gauge potentials and topological models. As a demonstration, we simulate a plaquette of the bosonic Creutz ladder. We characterize the lattice with scattering measurements, reconstructing the experimental Hamiltonian and observing important precursors of topological features including nonreciprocal transport and Aharonov-Bohm caging. This platform can be easily extended to larger lattices and different models involving other interactions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Chen, Q. -M.; Pfeiffer, M.; Partanen, M.; Fesquet, F.; Honasoge, K. E.; Kronowetter, F.; Nojiri, Y.; Renger, M.; Fedorov, K. G.; Marx, A.; Deppe, F.; Gross, R.

The scattering coefficients of superconducting microwave resonators: I. Transfer-matrix approach Miscellaneous

2021.

@misc{Chen2021g,

title = {The scattering coefficients of superconducting microwave resonators: I. Transfer-matrix approach},

author = {Q. -M. Chen and M. Pfeiffer and M. Partanen and F. Fesquet and K. E. Honasoge and F. Kronowetter and Y. Nojiri and M. Renger and K. G. Fedorov and A. Marx and F. Deppe and R. Gross},

url = {https://arxiv.org/abs/2109.07762},

year = {2021},

date = {2021-09-01},

abstract = {We describe a unified classical approach for analyzing the scattering coefficients of superconducting microwave resonators with a variety of geometries. To fill the gap between experiment and theory, we also consider the influences of small circuit asymmetry and the finite length of the feedlines, and describe a procedure to correct them in typical measurement results. We show that, similar to the transmission coefficient of a hanger-type resonator, the reflection coefficient of a necklace- or bridge-type resonator does also contain a reference point which can be used to characterize the electrical properties of a microwave resonator in a single measurement. Our results provide a comprehensive understanding of superconducting microwave resonators from the design concepts to the characterization details.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Chen, Q. -M.; Partanen, M.; Fesquet, F.; Honasoge, K. E.; Kronowetter, F.; Nojiri, Y.; Renger, M.; Fedorov, K. G.; Marx, A.; Deppe, F.; Gross, R.

The scattering coefficients of superconducting microwave resonators: II. System-bath approach Miscellaneous

2021.

@misc{Chen2021h,

title = {The scattering coefficients of superconducting microwave resonators: II. System-bath approach},

author = {Q. -M. Chen and M. Partanen and F. Fesquet and K. E. Honasoge and F. Kronowetter and Y. Nojiri and M. Renger and K. G. Fedorov and A. Marx and F. Deppe and R. Gross},

url = {https://arxiv.org/abs/2109.07766},

year = {2021},

date = {2021-09-01},

abstract = {We describe a unified quantum approach for analyzing the scattering coefficients of superconducting microwave resonators with a variety of geometries. We also generalize the method to a chain of resonators in either hanger- or necklace-type, and reveal interesting transport properties similar to a photonic crystal. It is shown that both the quantum and classical analyses provide consistent results, and they together form a solid basis for analyzing the decoherence effect in a general microwave resonator. These results pave the way for designing and applying superconducting microwave resonators in complex circuits, and should stimulate the interest of distinguishing different decoherence mechanisms of a resonator mode beyond free energy relaxation.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Girard, J. P.; Liu, W.; Kokkoniemi, R.; Visakorpi, E.; Govenius, J.; Möttönen, M.

Cryogenic power sensor enabling broad-band and traceable measurements Miscellaneous

2021.

@misc{Girard2021,

title = {Cryogenic power sensor enabling broad-band and traceable measurements},

author = {J. P. Girard and W. Liu and R. Kokkoniemi and E. Visakorpi and J. Govenius and M. Möttönen},

url = {https://arxiv.org/abs/2108.05101},

year = {2021},

date = {2021-08-01},

abstract = {Recently, great progress has been made in the field of ultrasensitive microwave detectors, reaching even the threshold for utilization in circuit quantum electrodynamics (cQED). However, these cryogenic sensors lack the ability to perform broad-band metrologically traceable power absorption measurements, which limits their scope of applications. Here, we demonstrate such measurements using an ultralow-noise nanobolometer supplemented by an additional direct-current (dc) input. The tracing of the absorbed power relies on comparing the response of the bolometer between radio frequency (rf) and dc heating powers traced through the Josephson voltage and quantum Hall resistance. To illustrate this technique, we demonstrate a fast calibration process of an attenuated input line over more than nine octaves of bandwidth with an rf heating power of -114 dBm and uncertainty down to 0.33 dB.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Puebla, R.; Ban, Y.; Haase, J.; Plenio, M.; Paternostro, M.; Casanova, J.

Versatile Atomic Magnetometry Assisted by Bayesian Inference Journal Article

In: Physical Review Applied, vol. 16, no. 2, pp. 024044, 2021.

@article{Puebla2021,

title = {Versatile Atomic Magnetometry Assisted by Bayesian Inference},

author = {R. Puebla and Y. Ban and J. Haase and M. Plenio and M. Paternostro and J. Casanova},

doi = {10.1103/physrevapplied.16.024044},

year = {2021},

date = {2021-08-01},

journal = {Physical Review Applied},

volume = {16},

number = {2},

pages = {024044},

publisher = {American Physical Society (APS)},

abstract = {Quantum sensors typically translate external fields into a periodic response whose frequency is then determined by analyses performed in Fourier space. This allows for a linear inference of the parameters that characterize external signals. In practice, however, quantum sensors are able to detect fields only in a narrow range of amplitudes and frequencies. A departure from this range, as well as the presence of significant noise sources and short detection times, lead to a loss of the linear relationship between the response of the sensor and the target field, thus limiting the working regime of the sensor. Here we address these challenges by means of a Bayesian inference approach that is tolerant to strong deviations from desired periodic responses of the sensor and is able to provide reliable estimates even with a very limited number of measurements. We demonstrate our method for an 171Yb+ trapped-ion quantum sensor but stress the general applicability of this approach to different systems.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Ban, Y.; Echanobe, J.; Ding, Y.; Puebla, R.; Casanova, J.

Neural-network-based parameter estimation for quantum detection Journal Article

In: Quantum Science and Technology, vol. 6, no. 4, pp. 045012, 2021.

@article{Ban2021c,

title = {Neural-network-based parameter estimation for quantum detection},

author = {Y. Ban and J. Echanobe and Y. Ding and R. Puebla and J. Casanova},

doi = {10.1088/2058-9565/ac16ed},

year = {2021},

date = {2021-08-01},

journal = {Quantum Science and Technology},

volume = {6},

number = {4},

pages = {045012},

publisher = {IOP Publishing},

abstract = {Artificial neural networks (NNs) bridge input data into output results by approximately encoding the function that relates them. This is achieved after training the network with a collection of known inputs and results leading to an adjustment of the neuron connections and biases. In the context of quantum detection schemes, NNs find a natural playground. In particular, in the presence of a target (e.g. an electromagnetic field), a quantum sensor delivers a response, i.e. the input data, which can be subsequently processed by a NN that outputs the target features. In this work we demonstrate that adequately trained NNs enable to characterize a target with (i) minimal knowledge of the underlying physical model (ii) in regimes where the quantum sensor presents complex responses and (iii) under a significant shot noise due to a reduced number of measurements. We exemplify the method with a development for 171Yb+ atomic sensors. However, our protocol is general, thus applicable to arbitrary quantum detection scenarios.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

García-Molina, P.; Martin, A.; Sanz, M.

Noise in Digital and Digital-Analog Quantum Computation Miscellaneous

2021.

@misc{GarciaMolina2021,

title = {Noise in Digital and Digital-Analog Quantum Computation},

author = {P. García-Molina and A. Martin and M. Sanz},

url = {https://arxiv.org/abs/2107.12969},

year = {2021},

date = {2021-07-01},

abstract = {Quantum computing makes use of quantum resources provided by the underlying quantum nature of matter to enhance classical computation. However, current Noisy Intermediate-Scale Quantum (NISQ) era in quantum computing is characterized by the use of quantum processors comprising from a few tens to, at most, few hundreds of physical qubits without implementing quantum error correction techniques. This limits the scalability in the implementation of quantum algorithms. Digital-analog quantum computing (DAQC) has been proposed as a more resilient alternative quantum computing paradigm to outperform digital quantum computation within the NISQ era framework. It arises from adding the flexibility provided by fast single-qubit gates to the robustness of analog quantum simulations. Here, we perform a careful comparison between digital and digital-analog paradigms under the presence of noise sources. The comparison is illustrated by comparing the performance of the quantum Fourier transform algorithm under a wide range of single- and two-qubit noise sources. Indeed, we obtain that, when the different noise channels usually present in superconducting quantum processors are considered, the fidelity of the QFT algorithm for the digital-analog paradigm outperforms the one obtained for the digital approach. Additionally, this difference grows when the size of the processor scales up, constituting consequently a sensible alternative paradigm in the NISQ era. Finally, we show how the DAQC paradigm can be adapted to quantum error mitigation techniques for canceling different noise sources, including the bang error.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Tobalina, A.; Munuera-Javaloy, C.; Torrontegui, E.; Muga, J. G.; Casanova, J.

Tailored Ion Beam for Precise Color Center Creation Miscellaneous

2021.

@misc{Tobalina2021,

title = {Tailored Ion Beam for Precise Color Center Creation},

author = {A. Tobalina and C. Munuera-Javaloy and E. Torrontegui and J. G. Muga and J. Casanova},

url = {https://arxiv.org/abs/2107.11249},

year = {2021},

date = {2021-07-01},

abstract = {We present a unitary quantum control scheme that produces a highly monochromatic ion beam from a Paul trap. Our protocol is implementable by supplying the segmented electrodes with voltages of the order of Volts, which mitigates the impact of fluctuating voltages in previous designs and leads to a low-dispersion beam of ions. Moreover, our proposal does not rely on sympathetically cooling the ions, which bypasses the need of loading different species in the trap -- namely, the propelled ion and, e.g., a $^40$Ca$^+$ atom able to exert sympathetic cooling -- incrementing the repetition rate of the launching procedure. Our scheme is based on an invariant operator linear in position and momentum, which enables us to control the average extraction energy and the outgoing momentum spread. In addition, we propose a sequential operation to tailor the transversal properties of the beam before the ejection to minimize the impact spot and to increase the lateral resolution of the implantation.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Peng, J.; Zheng, J.; Yu, J.; Tang, P.; Barrios, G. A.; Zhong, J.; Solano, E.; Albarrán-Arriagada, F.; Lamata, L.

One-Photon Solutions to the Multiqubit Multimode Quantum Rabi Model for Fast W -State Generation Journal Article

In: Physical Review Letters, vol. 127, no. 4, pp. 043604, 2021.

@article{Peng2021,

title = {One-Photon Solutions to the Multiqubit Multimode Quantum Rabi Model for Fast W -State Generation},

author = {J. Peng and J. Zheng and J. Yu and P. Tang and G. A. Barrios and J. Zhong and E. Solano and F. Albarrán-Arriagada and L. Lamata},

doi = {10.1103/physrevlett.127.043604},

year = {2021},

date = {2021-07-01},

journal = {Physical Review Letters},

volume = {127},

number = {4},

pages = {043604},

publisher = {American Physical Society (APS)},

abstract = {General solutions to the quantum Rabi model involve subspaces with an unbounded number of photons. However, for the multiqubit multimode case, we find special solutions with at most one photon for an arbitrary number of qubits and photon modes. Such solutions exist for arbitrary single qubit-photon coupling strength with constant eigenenergy, while still being qubit-photon entangled states. Taking advantage of their peculiarities and the reach of the ultrastrong coupling regime, we propose an adiabatic scheme for the fast and deterministic generation of a two-qubit Bell state and arbitrary single-photon multimode W states with nonadiabatic error less than 1%. Finally, we propose a superconducting circuit design to catch and release the W states, and shows the experimental feasibility of the multimode multiqubit quantum Rabi model.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Agustí, A.; García-Álvarez, L.; Solano, E.; Sabín, C.

Qubit motion as a microscopic model for the dynamical Casimir effect Journal Article

In: Physical Review A, vol. 103, no. 6, pp. 062201, 2021.

@article{Agusti2021,

title = {Qubit motion as a microscopic model for the dynamical Casimir effect},

author = {A. Agustí and L. García-Álvarez and E. Solano and C. Sabín},

doi = {10.1103/physreva.103.062201},

year = {2021},

date = {2021-06-01},

journal = {Physical Review A},

volume = {103},

number = {6},

pages = {062201},

publisher = {American Physical Society (APS)},

abstract = {The generation of photons from the vacuum by means of the movement of a mirror is known as the dynamical Casimir effect (DCE). In general, this phenomenon is effectively described by a field with time-dependent boundary conditions. Alternatively, we introduce a microscopic model of the DCE capable of capturing the essential features of the effect with no time-dependent boundary conditions. Besides the field, such a model comprises a subsystem representing the mirror's internal structure. In this work, we study one of the most straightforward mirror systems: a qubit moving in a cavity and coupled to one of the bosonic modes. We find that under certain conditions on the qubit's movement that do not depend on its physical properties, a large number of photons may be generated without changing the qubit state, as should be expected for a microscopic model of the mirror.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Asensio-Perea, R.; Parra-Rodriguez, A.; Kirchmair, G.; Solano, E.; Rico, E.

Chiral states and nonreciprocal phases in a Josephson junction ring Journal Article

In: Physical Review B, vol. 103, no. 22, pp. 224525, 2021.

@article{AsensioPerea2021,

title = {Chiral states and nonreciprocal phases in a Josephson junction ring},

author = {R. Asensio-Perea and A. Parra-Rodriguez and G. Kirchmair and E. Solano and E. Rico},

doi = {10.1103/physrevb.103.224525},

year = {2021},

date = {2021-06-01},

journal = {Physical Review B},

volume = {103},

number = {22},

pages = {224525},

publisher = {American Physical Society (APS)},

abstract = {In this work, we propose how to load and manipulate chiral states in a Josephson junction ring in the so-called transmon regime. We characterize these states by their symmetry properties under time-reversal and parity transformations. We describe an explicit protocol to load and detect the states within a realistic set of circuit parameters and show simulations that reveal the chiral nature. Finally, we explore the utility of these states in quantum technological nonreciprocal devices.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Miranda, E. R.; Venkatesh, S.; Hernani-Morales, C.; Lamata, L.; Martín-Guerrero, J. D.; Solano, E.

Quantum Brain Networks: a Perspective Miscellaneous

2021.

@misc{Miranda2021,

title = {Quantum Brain Networks: a Perspective},

author = {E. R. Miranda and S. Venkatesh and C. Hernani-Morales and L. Lamata and J. D. Martín-Guerrero and E. Solano},

url = {https://arxiv.org/abs/2106.12295},

year = {2021},

date = {2021-06-01},

abstract = {We propose Quantum Brain Networks (QBraiNs) as a new interdisciplinary field integrating knowledge and methods from neurotechnology, artificial intelligence, and quantum computing. The objective is to develop an enhanced connectivity between the human brain and quantum computers for a variety of disruptive applications. We foresee the emergence of hybrid classical-quantum networks of wetware and hardware nodes, mediated by machine learning techniques and brain-machine interfaces. QBraiNs will harness and transform in unprecedented ways arts, science, technologies, and entrepreneurship, in particular activities related to medicine, Internet of humans, intelligent devices, sensorial experience, gaming, Internet of things, crypto trading, and business.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Costa, N. F.; Omar, Y.; Sultanov, A.; Paraoanu, G. S.

Benchmarking machine learning algorithms for adaptive quantum phase estimation with noisy intermediate-scale quantum sensors Journal Article

In: EPJ Quantum Technology, vol. 8, no. 1, 2021.

@article{Costa2021,

title = {Benchmarking machine learning algorithms for adaptive quantum phase estimation with noisy intermediate-scale quantum sensors},

author = {N. F. Costa and Y. Omar and A. Sultanov and G. S. Paraoanu},

doi = {10.1140/epjqt/s40507-021-00105-y},

year = {2021},

date = {2021-06-01},

journal = {EPJ Quantum Technology},

volume = {8},

number = {1},

publisher = {Springer Science and Business Media LLC},

abstract = {Quantum phase estimation is a paradigmatic problem in quantum sensing and metrology. Here we show that adaptive methods based on classical machine learning algorithms can be used to enhance the precision of quantum phase estimation when noisy non-entangled qubits are used as sensors. We employ the Differential Evolution (DE) and Particle Swarm Optimization (PSO) algorithms to this task and we identify the optimal feedback policies which minimize the Holevo variance. We benchmark these schemes with respect to scenarios that include Gaussian and Random Telegraph fluctuations as well as reduced Ramsey-fringe visibility due to decoherence. We discuss their robustness against noise in connection with real experimental setups such as Mach-Zehnder interferometry with optical photons and Ramsey interferometry in trapped ions, superconducting qubits and nitrogen-vacancy (NV) centers in diamond.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Pan, C. -Y.; Hao, M.; Barraza, N.; Solano, E.; Albarrán-Arriagada, F.

Experimental semi-autonomous eigensolver using reinforcement learning Journal Article

In: Scientific Reports, vol. 11, no. 1, 2021.

@article{Pan2021a,

title = {Experimental semi-autonomous eigensolver using reinforcement learning},

author = {C. -Y. Pan and M. Hao and N. Barraza and E. Solano and F. Albarrán-Arriagada},

doi = {10.1038/s41598-021-90534-7},

year = {2021},

date = {2021-06-01},

journal = {Scientific Reports},

volume = {11},

number = {1},

publisher = {Springer Science and Business Media LLC},

abstract = {The characterization of observables, expressed via Hermitian operators, is a crucial task in quantum mechanics. For this reason, an eigensolver is a fundamental algorithm for any quantum technology. In this work, we implement a semi-autonomous algorithm to obtain an approximation of the eigenvectors of an arbitrary Hermitian operator using the IBM quantum computer. To this end, we only use single-shot measurements and pseudo-random changes handled by a feedback loop, reducing the number of measures in the system. Due to the classical feedback loop, this algorithm can be cast into the reinforcement learning paradigm. Using this algorithm, for a single-qubit observable, we obtain both eigenvectors with fidelities over 0.97 with around 200 single-shot measurements. For two-qubits observables, we get fidelities over 0.91 with around 1500 single-shot measurements for the four eigenvectors, which is a comparatively low resource demand, suitable for current devices. This work is useful to the development of quantum devices able to decide with partial information, which helps to implement future technologies in quantum artificial intelligence.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Yu, J.; Cárdenas-López, F. A.; Andersen, C. K.; Solano, E.; Parra-Rodriguez, A.

Charge qubits in the ultrastrong coupling regime Miscellaneous

2021.

@misc{Yu2021b,

title = {Charge qubits in the ultrastrong coupling regime},

author = {J. Yu and F. A. Cárdenas-López and C. K. Andersen and E. Solano and A. Parra-Rodriguez},

url = {https://arxiv.org/abs/2105.06851},

year = {2021},

date = {2021-05-01},

abstract = {We study the feasibility of reaching the ultrastrong (USC) and deep-strong coupling (DSC) regimes of light-matter interaction, in particular at resonance condition, with a superconducting charge qubit, also known as Cooper-Pair box (CPB). We show that by shunting the charge qubit with a high-impedance LC-circuit, one can maximally reach both USC and DSC regimes exceeding the classical upper bound $|g|łeq sqrtømega_qømega_r/2$ between two harmonic systems with frequencies $ømega_q$ and $ømega_r$. In our case, the fundamental model corresponds to an enhanced quantum Rabi model, which contains a displacement field operator that breaks its internal parity symmetry. Furthermore, we consider a multipartite device consisting of two CPBs ultrastrongly coupled to an oscillator as a mediator and study a quantum state transfer protocol between a pair of transmon qubits, all of them subjected to local incoherent noise channels with realistic parameters. This work opens the door for studying light-matter interactions beyond the quantum Rabi model at extreme coupling strengths, providing a new building block for applications within quantum computation and quantum information processing.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Dassonneville, R.; Assouly, R.; Peronnin, T.; Clerk, A.; Bienfait, A.; Huard, B.

Dissipative Stabilization Of Squeezing Beyond 3 dB In A Microwave Mode Journal Article

In: PRX Quantum, vol. 2, no. 2, pp. 020323, 2021.

@article{Dassonneville2021,

title = {Dissipative Stabilization Of Squeezing Beyond 3 dB In A Microwave Mode},

author = {R. Dassonneville and R. Assouly and T. Peronnin and A. Clerk and A. Bienfait and B. Huard},

doi = {10.1103/prxquantum.2.020323},

year = {2021},

date = {2021-05-01},

journal = {PRX Quantum},

volume = {2},

number = {2},

pages = {020323},

publisher = {American Physical Society (APS)},

abstract = {While a propagating state of light can be generated with arbitrary squeezing by pumping a parametric resonator, the intraresonator state is limited to 3 dB of squeezing. Here, we implement a reservoir-engineering method to surpass this limit using superconducting circuits. Two-tone pumping of a three-wave-mixing element implements an effective coupling to a squeezed bath, which stabilizes a squeezed state inside the resonator. Using an ancillary superconducting qubit as a probe allows us to perform a direct Wigner tomography of the intraresonator state. The raw measurement provides a lower bound on the squeezing at about 6.7pm 0.2 dB below the zero-point level. Further, we show how to correct for resonator evolution during the Wigner tomography and obtain a squeezing as high as 8.2pm 0.8 dB. Moreover, this level of squeezing is achieved with a purity of 0.91pm 0.09.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Gonzalez-Raya, T.; Asensio-Perea, R.; Martin, A.; Céleri, L. C.; Sanz, M.; Lougovski, P.; Dumitrescu, E. F.

Digital-Analog Quantum Simulations Using the Cross-Resonance Effect Journal Article

In: PRX Quantum, vol. 2, no. 2, pp. 020328, 2021.

@article{GonzalezRaya2021,

title = {Digital-Analog Quantum Simulations Using the Cross-Resonance Effect},

author = {T. Gonzalez-Raya and R. Asensio-Perea and A. Martin and L. C. Céleri and M. Sanz and P. Lougovski and E. F. Dumitrescu},

doi = {10.1103/prxquantum.2.020328},

year = {2021},

date = {2021-05-01},

journal = {PRX Quantum},

volume = {2},

number = {2},

pages = {020328},

publisher = {American Physical Society (APS)},

abstract = {Digital-analog quantum computation aims to reduce the currently infeasible resource requirements needed for near-term quantum information processing by replacing sequences of one- and two-qubit gates with a unitary transformation generated by the systems' underlying Hamiltonian. Inspired by this paradigm, we consider superconducting architectures and extend the cross-resonance effect, up to first order in perturbation theory, from a two-qubit interaction to an analog Hamiltonian acting on one-dimensional (1D) chains and two-dimensional (2D) square lattices, which, in an appropriate reference frame, results in a purely two-local Hamiltonian. By augmenting the analog Hamiltonian dynamics with single-qubit gates we show how one may generate a larger variety of distinct analog Hamiltonians. We then synthesize unitary sequences, in which we toggle between the various analog Hamiltonians as needed, simulating the dynamics of Ising, XY, and Heisenberg spin models. Our dynamics simulations are Trotter error-free for the Ising and XY models in 1D. We also show that the Trotter errors for 2D XY and 1D Heisenberg chains are reduced, with respect to a digital decomposition, by a constant factor. In order to realize these important near-term speedups, we discuss the practical considerations needed to accurately characterize and calibrate our analog Hamiltonians for use in quantum simulations. We conclude with a discussion of how the Hamiltonian toggling techniques could be extended to derive new analog Hamiltonians, which may be of use in more complex digital-analog quantum simulations for various models of interacting spins.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Ban, Y.; Echanobe, J.; Torrontegui, E.; Casanova, J.

Mutual Reinforcement between Neural Networks and Quantum Physics Miscellaneous

2021.

@misc{Ban2021a,

title = {Mutual Reinforcement between Neural Networks and Quantum Physics},

author = {Y. Ban and J. Echanobe and E. Torrontegui and J. Casanova},

url = {https://arxiv.org/abs/2105.13273},

year = {2021},

date = {2021-05-01},

journal = {Proceedings of the XIX Conference of the Spanish Association for Artificial Intelligence (2021). ISBN 978-84-09-30514-8},

abstract = {Quantum machine learning emerges from the symbiosis of quantum mechanics and machine learning. In particular, the latter gets displayed in quantum sciences as: (i) the use of classical machine learning as a tool applied to quantum physics problems, (ii) or the use of quantum resources such as superposition, entanglement, or quantum optimization protocols to enhance the performance of classification and regression tasks compare to their classical counterparts. This paper reviews examples in these two scenarios. On the one hand, a classical neural network is applied to design a new quantum sensing protocol. On the other hand, the design of a quantum neural network based on the dynamics of a quantum perceptron with the application of shortcuts to adiabaticity gives rise to a short operation time and robust performance. These examples demonstrate the mutual reinforcement of both neural networks and quantum physics.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Ban, Yue; Torrontegui, E.; Casanova, J.

Quantum neural networks with multi-qubit potentials Miscellaneous

2021.

@misc{Ban2021b,

title = {Quantum neural networks with multi-qubit potentials},

author = {Yue Ban and E. Torrontegui and J. Casanova},

url = {https://arxiv.org/abs/2105.02756},

year = {2021},

date = {2021-05-01},

abstract = {We propose quantum neural networks that include multi-qubit interactions in the neural potential leading to a reduction of the network depth without losing approximative power. We show that the presence of multi-qubit potentials in the quantum perceptrons enables more efficient information processing tasks such as XOR gate implementation and prime numbers search, while it also provides a depth reduction to construct distinct entangling quantum gates like CNOT, Toffoli, and Fredkin. This simplification in the network architecture paves the way to address the connectivity challenge to scale up a quantum neural network while facilitates its training.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Ding, Y.; Ban, Y.; Martin-Guerrero, J. D.; Solano, E.; Casanova, J.; Chen, X.

Breaking adiabatic quantum control with deep learning Journal Article

In: Physical Review A, vol. 103, no. 4, pp. l040401, 2021.

@article{Ding2021a,

title = {Breaking adiabatic quantum control with deep learning},

author = {Y. Ding and Y. Ban and J. D. Martin-Guerrero and E. Solano and J. Casanova and X. Chen},

doi = {10.1103/physreva.103.l040401},

year = {2021},

date = {2021-04-01},

journal = {Physical Review A},

volume = {103},

number = {4},

pages = {l040401},

publisher = {American Physical Society (APS)},

abstract = {In the noisy intermediate-scale quantum era, optimal digitized pulses are requisite for efficient quantum control. This goal is translated into dynamic programming, in which a deep reinforcement learning (DRL) agent is gifted. As a reference, shortcuts to adiabaticity (STA) provide analytical approaches to adiabatic speedup by pulse control. Here, we select the single-component control of qubits, resembling the ubiquitous two-level Landau-Zener problem for gate operation. We aim at obtaining fast and robust digital pulses by combining the STA and DRL algorithm. In particular, we find that DRL leads to robust digital quantum control with the operation time bounded by quantum speed limits dictated by STA. In addition, we demonstrate that robustness against systematic errors can be achieved by DRL without any input from STA. Our results introduce a general framework of digital quantum control, leading to a promising enhancement in quantum information processing.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Munuera-Javaloy, C.; Puebla, R.; Casanova, J.

Dynamical decoupling methods in nanoscale NMR Journal Article

In: Europhysics Letters, vol. 134, pp. 30001, 2021.

@article{MunueraJavaloy2021,

title = {Dynamical decoupling methods in nanoscale NMR},

author = {C. Munuera-Javaloy and R. Puebla and J. Casanova},

doi = {10.1209/0295-5075/ac0ed1},

year = {2021},

date = {2021-04-01},

journal = {Europhysics Letters},

volume = {134},

pages = {30001},

abstract = {Nuclear magnetic resonance (NMR) schemes can be applied to micron-, and nanometer-sized samples by the aid of quantum sensors such as nitrogen-vacancy (NV) color centers in diamond. These minute devices allow for magnetometry of nuclear spin ensembles with high spatial and frequency resolution at ambient conditions, thus having a clear impact in different areas such as chemistry, biology, medicine, and material sciences. In practice, NV quantum sensors are driven by microwave (MW) control fields with a twofold objective: On the one hand, MW fields bridge the energy gap between NV and nearby nuclei which enables a coherent and selective coupling among them while, on the other hand, MW fields remove environmental noise on the NV leading to enhanced interrogation time. In this work we review distinct MW radiation patterns, or dynamical decoupling techniques, for nanoscale NMR applications.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Salari, V.

Unconditional Microwave Quantum Teleportation of Gaussian States in Lossy Environments Miscellaneous

2021.

@misc{Salari2021,

title = {Unconditional Microwave Quantum Teleportation of Gaussian States in Lossy Environments},

author = {V. Salari},

url = {https://arxiv.org/abs/2103.02607},

year = {2021},

date = {2021-03-01},

abstract = {Here, a physical formalism is proposed for an unconditional microwave quantum teleportation of Gaussian states via two-mode squeezed states in lossy environments. The proposed formalism is controllable to be used in both the fridge and free space in case of entanglement between two parties survives. Some possible experimental parameters are estimated for the teleportation of microwave signals with a frequency of 5GHz based on the proposed physical framework. This would be helpful for superconducting inter- and intra-fridge quantum communication as well as open-air quantum microwave communication, which can be applied to quantum local area networks (QLANs) and distributed quantum computing protocols.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Fischer, M.; Chen, Q. -M.; Besson, C.; Eder, P.; Goetz, J.; Pogorzalek, S.; Renger, M.; Xie, E.; Hartmann, M. J.; Fedorov, K. G.; Marx, A.; Deppe, F.; Gross, R.

In situ tunable nonlinearity and competing signal paths in coupled superconducting resonators Journal Article

In: Physical Review B, vol. 103, no. 9, pp. 094515, 2021.

@article{Fischer2021,

title = {In situ tunable nonlinearity and competing signal paths in coupled superconducting resonators},

author = {M. Fischer and Q. -M. Chen and C. Besson and P. Eder and J. Goetz and S. Pogorzalek and M. Renger and E. Xie and M. J. Hartmann and K. G. Fedorov and A. Marx and F. Deppe and R. Gross},

doi = {10.1103/physrevb.103.094515},

year = {2021},

date = {2021-03-01},

journal = {Physical Review B},

volume = {103},

number = {9},

pages = {094515},

publisher = {American Physical Society (APS)},

abstract = {We have fabricated and studied a system of two tunable and coupled nonlinear superconducting resonators. The nonlinearity is introduced by galvanically coupled dc superconducting quantum interference devices. We simulate the system response by means of a circuit model, which includes an additional signal path introduced by the electromagnetic environment. Furthermore, we present two methods allowing us to experimentally determine the nonlinearity. First, we fit the measured frequency and flux dependence of the transmission data to simulations based on the equivalent circuit model. Second, we fit the power dependence of the transmission data to a model that is predicted by the nonlinear equation of motion describing the system. Our results show that we are able to tune the nonlinearity of the resonators by almost two orders of magnitude via an external coil and two on-chip antennas. The studied system represents a basic building block for larger systems, allowing for quantum simulations of bosonic many-body systems with a larger number of lattice sites.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Céleri, L. C.; Huerga, D.; Albarrán-Arriagada, F.; Solano, E.; Sanz, M.

Digital-analog quantum simulation of fermionic models Miscellaneous

2021.

@misc{Celeri2021,

title = {Digital-analog quantum simulation of fermionic models},

author = {L. C. Céleri and D. Huerga and F. Albarrán-Arriagada and E. Solano and M. Sanz},

url = {https://arxiv.org/abs/2103.15689},

year = {2021},

date = {2021-03-01},

abstract = {Simulating quantum many-body systems is a highly demanding task since the required resources grow exponentially with the dimension of the system. In the case of fermionic systems, this is even harder since nonlocal interactions emerge due to the antisymmetric character of the fermionic wave function. Here, we introduce a digital-analog quantum algorithm to simulate a wide class of fermionic Hamiltonians including the paradigmatic Fermi-Hubbard model. These digital-analog methods allow quantum algorithms to run beyond digital versions via an efficient use of coherence time. Furthermore, we exemplify our techniques with a low-connected architecture for realistic digital-analog implementations of specific fermionic models.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Ban, Y.; Chen, X.; Torrontegui, E.; Solano, E.; Casanova, J.

Speeding up quantum perceptron via shortcuts to adiabaticity Journal Article

In: Scientific Reports, vol. 11, no. 1, 2021.

@article{Ban2021,

title = {Speeding up quantum perceptron via shortcuts to adiabaticity},

author = {Y. Ban and X. Chen and E. Torrontegui and E. Solano and J. Casanova},

doi = {10.1038/s41598-021-85208-3},

year = {2021},

date = {2021-03-01},

journal = {Scientific Reports},

volume = {11},

number = {1},

publisher = {Springer Science and Business Media LLC},

abstract = {The quantum perceptron is a fundamental building block for quantum machine learning. This is a multidisciplinary field that incorporates abilities of quantum computing, such as state superposition and entanglement, to classical machine learning schemes. Motivated by the techniques of shortcuts to adiabaticity, we propose a speed-up quantum perceptron where a control field on the perceptron is inversely engineered leading to a rapid nonlinear response with a sigmoid activation function. This results in faster overall perceptron performance compared to quasi-adiabatic protocols, as well as in enhanced robustness against imperfections in the controls.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Pires, D. P.; Modi, K.; Céleri, L. C.

Bounding generalized relative entropies: Nonasymptotic quantum speed limits Journal Article

In: Physical Review E, vol. 103, no. 3, pp. 032105, 2021.

@article{Pires2021,

title = {Bounding generalized relative entropies: Nonasymptotic quantum speed limits},

author = {D. P. Pires and K. Modi and L. C. Céleri},

doi = {10.1103/physreve.103.032105},

year = {2021},

date = {2021-03-01},

journal = {Physical Review E},

volume = {103},

number = {3},

pages = {032105},

publisher = {American Physical Society (APS)},

abstract = {Information theory has become an increasingly important research field to better understand quantum mechanics. Noteworthy, it covers both foundational and applied perspectives, also offering a common technical language to study a variety of research areas. Remarkably, one of the key information-theoretic quantities is given by the relative entropy, which quantifies how difficult is to tell apart two probability distributions, or even two quantum states. Such a quantity rests at the core of fields like metrology, quantum thermodynamics, quantum communication, and quantum information. Given this broadness of applications, it is desirable to understand how this quantity changes under a quantum process. By considering a general unitary channel, we establish a bound on the generalized relative entropies (Renyi and Tsallis) between the output and the input of the channel. As an application of our bounds, we derive a family of quantum speed limits based on relative entropies. Possible connections between this family with thermodynamics, quantum coherence, asymmetry, and single-shot information theory are briefly discussed.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Dong, L.; Arrazola, I.; Chen, X.; Casanova, J.

Phase-Adaptive Dynamical Decoupling Methods for Robust Spin-Spin Dynamics in Trapped Ions Journal Article

In: Physical Review Applied, vol. 15, no. 3, pp. 034055, 2021.

@article{Dong2021,

title = {Phase-Adaptive Dynamical Decoupling Methods for Robust Spin-Spin Dynamics in Trapped Ions},

author = {L. Dong and I. Arrazola and X. Chen and J. Casanova},

doi = {10.1103/physrevapplied.15.034055},

year = {2021},

date = {2021-03-01},

journal = {Physical Review Applied},

volume = {15},

number = {3},

pages = {034055},

publisher = {American Physical Society (APS)},

abstract = {Quantum platforms based on trapped ions are the main candidates to build a quantum hardware with computational capacities that largely surpass those of classical devices. Among the available control techniques in these setups, pulsed dynamical decoupling (pulsed DD) has been revealed as a useful method to process the information encoded in ion registers, whilst minimizing the environmental noise over them. In this work, we incorporate a pulsed DD technique that uses random pulse phases, or correlated pulse phases, to significantly enhance the robustness of entangling spin-spin dynamics in trapped ions. This procedure was originally conceived in the context of nuclear magnetic resonance for nuclear spin detection purposes, and here we demonstrate that the same principles apply for robust quantum-information processing in trapped-ion settings.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Bugalho, L.; Coutinho, B. C.; Omar, Y.

Distributing Multipartite Entanglement over Noisy Quantum Networks Miscellaneous

2021.

@misc{Bugalho2021,

title = {Distributing Multipartite Entanglement over Noisy Quantum Networks},

author = {L. Bugalho and B. C. Coutinho and Y. Omar},

url = {https://arxiv.org/abs/2103.14759},

year = {2021},

date = {2021-03-01},

abstract = {A quantum internet aims at harnessing networked quantum technologies, namely by distributing bipartite entanglement between distant nodes. However, multipartite entanglement between the nodes may empower the quantum internet for additional or better applications for communications, sensing, and computation. In this work, we present an algorithm for generating multipartite entanglement between different nodes of a quantum network with noisy quantum repeaters and imperfect quantum memories, where the links are entangled pairs. Our algorithm is optimal for GHZ states with 3 qubits, maximising simultaneously the final state fidelity and the rate of entanglement distribution. Furthermore, we determine the conditions yielding this simultaneous optimality for GHZ states with a higher number of qubits, and for other types of multipartite entanglement. Our algorithm is general also in the sense that it can optimise simultaneously arbitrary parameters. This work opens the way to optimally generate multipartite quantum correlations over noisy quantum networks, an important resource for distributed quantum technologies.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Gatti, G.; Huerga, D.; Solano, E.; Sanz, M.

Random access codes via quantum contextual redundancy Miscellaneous

2021.

@misc{Gatti2021,

title = {Random access codes via quantum contextual redundancy},

author = {G. Gatti and D. Huerga and E. Solano and M. Sanz},

url = {https://arxiv.org/abs/2103.01204},

year = {2021},

date = {2021-03-01},

abstract = {We propose a protocol to encode classical bits in the measurement statistics of a set of parity observables, leveraging quantum contextual relations for a random access code task. The intrinsic information redundancy of quantum contexts allows for a posterior decoding protocol that requires few samples when encoding the information in a set of highly entangled states, which can be generated by a discretely-parametrized quantum circuit. Applications of this protocol include algorithms involving storage of large amounts of data but requiring only partial retrieval of the information, as is the case of decision trees. This classical-to-quantum encoding is a compression protocol for more than $18$ qubits and shows quantum advantage over state-of-the-art information storage capacity for more than $44$ qubits. In particular, systems above $100$ qubits would be sufficient to encode a brute force solution for games of chess-like complexity.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Hegade, N. N.; Paul, K.; Ding, Y.; Sanz, M.; Albarrán-Arriagada, F.; Solano, E.; Chen, X.

Shortcuts to Adiabaticity in Digitized Adiabatic Quantum Computing Journal Article

In: Physical Review Applied, vol. 15, no. 2, pp. 024038, 2021.

@article{Hegade2021,

title = {Shortcuts to Adiabaticity in Digitized Adiabatic Quantum Computing},

author = {N. N. Hegade and K. Paul and Y. Ding and M. Sanz and F. Albarrán-Arriagada and E. Solano and X. Chen},

doi = {10.1103/physrevapplied.15.024038},

year = {2021},

date = {2021-02-01},

journal = {Physical Review Applied},

volume = {15},

number = {2},

pages = {024038},

publisher = {American Physical Society (APS)},

abstract = {Shortcuts to adiabaticity are well-known methods for controlling the quantum dynamics beyond the adiabatic criteria, where counterdiabatic (CD) driving provides a promising means to speed up quantum many-body systems. In this work, we show the applicability of CD driving to enhance the digitized adiabatic quantum computing paradigm in terms of fidelity and total simulation time. We study the state evolution of an Ising spin chain using the digitized version of the standard CD driving and its variants derived from the variational approach. We apply this technique in the preparation of Bell and Greenberger-Horne-Zeilinger states with high fidelity using a very shallow quantum circuit. We implement this proposal on the IBM quantum computer, proving its usefulness for the speed up of adiabatic quantum computing in noisy intermediate-scale quantum devices.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Martin, A.; Candelas, B.; Rodríguez-Rozas, Á.; Martín-Guerrero, J. D.; Chen, X.; Lamata, L.; Orús, R.; Solano, E.; Sanz, M.

Toward pricing financial derivatives with an IBM quantum computer Journal Article

In: Physical Review Research, vol. 3, no. 1, pp. 013167, 2021.

@article{Martin2021,

title = {Toward pricing financial derivatives with an IBM quantum computer},

author = {A. Martin and B. Candelas and Á. Rodríguez-Rozas and J. D. Martín-Guerrero and X. Chen and L. Lamata and R. Orús and E. Solano and M. Sanz},

doi = {10.1103/physrevresearch.3.013167},

year = {2021},

date = {2021-02-01},

journal = {Physical Review Research},

volume = {3},

number = {1},

pages = {013167},

publisher = {American Physical Society (APS)},

abstract = {Pricing interest-rate financial derivatives is a major problem in finance, in which it is crucial to accurately reproduce the time evolution of interest rates. Several stochastic dynamics have been proposed in the literature to model either the instantaneous interest rate or the instantaneous forward rate. A successful approach to model the latter is the celebrated Heath-Jarrow-Morton framework, in which its dynamics is entirely specified by volatility factors. In its multifactor version, this model considers several noisy components to capture at best the dynamics of several time-maturing forward rates. However, as no general analytical solution is available, there is a trade-off between the number of noisy factors considered and the computational time to perform a numerical simulation. Here, we employ the quantum principal component analysis to reduce the number of noisy factors required to accurately simulate the time evolution of several time-maturing forward rates. The principal components are experimentally estimated with the five-qubit IBMQX2 quantum computer for 2x2 and 3x3 cross-correlation matrices, which are based on historical data for two and three time-maturing forward rates. This paper is a step towards the design of a general quantum algorithm to fully simulate on quantum computers the Heath-Jarrow-Morton model for pricing interest-rate financial derivatives. It shows indeed that practical applications of quantum computers in finance will be achievable in the near future.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Barrios, G. A.; Albarrán-Arriagada, F.; Peña, F. J.; Solano, E.; Retamal, J. C.

Light-matter quantum Otto engine in finite time Miscellaneous

2021.

@misc{Barrios2021,

title = {Light-matter quantum Otto engine in finite time},

author = {G. A. Barrios and F. Albarrán-Arriagada and F. J. Peña and E. Solano and J. C. Retamal},

url = {https://arxiv.org/abs/2102.10559},

year = {2021},

date = {2021-02-01},

abstract = {We study a quantum Otto engine at finite time, where the working substance is composed of a two-level system interacting with a harmonic oscillator, described by the quantum Rabi model. We obtain the limit cycle and calculate the total work extracted, efficiency, and power of the engine by numerically solving the master equation describing the open system dynamics. We relate the total work extracted and the efficiency at maximum power with the quantum correlations embedded in the working substance, which we consider through entanglement of formation and quantum discord. Interestingly, we find that the engine can overcome the Curzon-Ahlborn efficiency when the working substance is in the ultrastrong coupling regime. This high-efficiency regime roughly coincides with the cases where the entanglement in the working substance experiences the greatest reduction in the hot isochoric stage. Our results highlight the efficiency performance of correlated working substances for quantum heat engines.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Ding, Y.; Chen, X.; Lamata, L.; Solano, E.; Sanz, M.

Implementation of a Hybrid Classical-Quantum Annealing Algorithm for Logistic Network Design Journal Article

In: SN Computer Science, vol. 2, no. 2, 2021.

@article{Ding2021c,

title = {Implementation of a Hybrid Classical-Quantum Annealing Algorithm for Logistic Network Design},

author = {Y. Ding and X. Chen and L. Lamata and E. Solano and M. Sanz},

doi = {10.1007/s42979-021-00466-2},

year = {2021},

date = {2021-02-01},

journal = {SN Computer Science},

volume = {2},

number = {2},

publisher = {Springer Science and Business Media LLC},

abstract = {The logistic network design is an abstract optimization problem that, under the assumption of minimal cost, seeks the optimal configuration of the supply chain's infrastructures and facilities based on customer demand. Key economic decisions are taken about the location, number, and size of manufacturing facilities and warehouses based on the optimal solution. Therefore, improvements in the methods to address this question, which is known to be in the NP-hard complexity class, would have relevant financial consequences. Here, we implement in the D-Wave quantum annealer a hybrid classical-quantum annealing algorithm. The cost function with constraints is translated to a spin Hamiltonian, whose ground state encodes the searched result. As a benchmark, we measure the accuracy of results for a set of paradigmatic problems against the optimal published solutions (the error is on average below 1%), and the performance is compared against the classical algorithm, showing a remarkable reduction in the number of iterations. This work shows that state-of-the-art quantum annealers may codify and solve relevant supply-chain problems even still far from useful quantum supremacy.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Gonzalez-Conde, J.; Rodríguez-Rozas, Á.; Solano, E.; Sanz, M.

Simulating option price dynamics with exponential quantum speedup Miscellaneous

2021.

@misc{GonzalezConde2021,

title = {Simulating option price dynamics with exponential quantum speedup},

author = {J. Gonzalez-Conde and Á. Rodríguez-Rozas and E. Solano and M. Sanz},

url = {https://arxiv.org/abs/2101.04023},

year = {2021},

date = {2021-01-01},

abstract = {Pricing financial derivatives, in particular European-style options at different time-maturities and strikes, is a relevant financial problem. The dynamics describing the price of vanilla options when constant volatilities and interest rates are assumed, is governed by the Black-Scholes model, a linear parabolic partial differential equation with terminal value given by the pay-off of the option contract and no additional boundary conditions. Here, we present a digital quantum algorithm to solve Black-Scholes equation on a quantum computer for a wide range of relevant financial parameters by mapping it to the Schrödinger equation. The non-Hermitian nature of the resulting Hamiltonian is solved by embedding the dynamics into an enlarged Hilbert space, which makes use of only one additional ancillary qubit. Moreover, we employ a second ancillary qubit to transform initial condition into periodic boundary conditions, which substantially improves the stability and performance of the protocol. This algorithm shows a feasible approach for pricing financial derivatives on a digital quantum computer based on Hamiltonian simulation, technique which differs from those based on Monte Carlo simulations to solve the stochastic counterpart of the Black Scholes equation. Our algorithm remarkably provides an exponential speedup since the terms in the Hamiltonian can be truncated by a polynomial number of interactions while keeping the error bounded. We report expected accuracy levels comparable to classical numerical algorithms by using 10 qubits and 94 entangling gates on a fault-tolerant quantum computer, and an expected success probability of the post-selection procedure due to the embedding protocol above 60%.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Pires, D.; Bargassa, P.; Seixas, J.; Omar, Y.

A Digital Quantum Algorithm for Jet Clustering in High-Energy Physics Journal Article

In: 2021.

@article{Pires2021a,

title = {A Digital Quantum Algorithm for Jet Clustering in High-Energy Physics},

author = {D. Pires and P. Bargassa and J. Seixas and Y. Omar},

url = {https://arxiv.org/abs/2101.05618},

year = {2021},

date = {2021-01-01},

abstract = {Experimental High-Energy Physics (HEP), especially the Large Hadron Collider (LHC) programme at the European Organization for Nuclear Research (CERN), is one of the most computationally intensive activities in the world. This demand is set to increase significantly with the upcoming High-Luminosity LHC (HL-LHC), and even more in future machines, such as the Future Circular Collider (FCC). As a consequence, event reconstruction, and in particular jet clustering, is bound to become an even more daunting problem, thus challenging present day computing resources. In this work, we present the first digital quantum algorithm to tackle jet clustering, opening the way for digital quantum processors to address this challenging problem. Furthermore, we show that, at present and future collider energies, our algorithm has comparable, yet generally lower complexity relative to the classical state-of-the-art $k_t$ clustering algorithm.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Deppe, F.; Xie, E.; Fedorov, K. G.; Andersson, G.; Müller, J.; Marx, A.; Gross, R.

RF Antenna Design for 3D Quantum Memories Inproceedings

In: 2021 International Applied Computational Electromagnetics Society Symposium (ACES), pp. 1–4, 2021.

@inproceedings{Deppe2021,

title = {RF Antenna Design for 3D Quantum Memories},

author = {F. Deppe and E. Xie and K. G. Fedorov and G. Andersson and J. Müller and A. Marx and R. Gross},

year = {2021},

date = {2021-01-01},

booktitle = {2021 International Applied Computational Electromagnetics Society Symposium (ACES)},

pages = {1--4},

abstract = {A quantum memory has to meet the conflicting requirements of strong coupling for fast readout and weak coupling for long storage. Multimode rectangular superconducting 3D cavities are known to satisfy both properties. Here, we systematically study the external coupling to the two lowest-frequency modes of an aluminum cavity. First, we introduce a general analytical scheme to describe the capacitive coupling of the antenna pin and validate this model experimentally. On this basis, we engineer an antenna which is overcoupled to the first mode, but undercoupled to the second mode.},

keywords = {},

pubstate = {published},

tppubtype = {inproceedings}

}

### 2020

Pires, D.; Omar, Y.; Seixas, J.

Adiabatic Quantum Algorithm for Multijet Clustering in High Energy Physics Miscellaneous

2020.

@misc{Pires2020,

title = {Adiabatic Quantum Algorithm for Multijet Clustering in High Energy Physics},

author = {D. Pires and Y. Omar and J. Seixas},

url = {https://arxiv.org/abs/2012.14514},

year = {2020},

date = {2020-12-01},

abstract = {The currently predicted increase in computational demand for the upcoming High-Luminosity Large Hadron Collider (HL-LHC) event reconstruction, and in particular jet clustering, is bound to challenge present day computing resources, becoming an even more complex combinatorial problem. In this paper, we show that quantum annealing can tackle dijet event clustering by introducing a novel quantum annealing binary clustering algorithm. The benchmarked efficiency is of the order of $96%$, thus yielding substantial improvements over the current quantum state-of-the-art. Additionally, we also show how to generalize the proposed objective function into a more versatile form, capable of solving the clustering problem in multijet events.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Oliveira, A.; Gomes, R.; Brasil, V.; Silva, N. R.; Céleri, L.; Ribeiro, P. S.

Full thermalization of a photonic qubit Journal Article

In: Physics Letters A, vol. 384, no. 36, pp. 126933, 2020.

@article{Oliveira2020,

title = {Full thermalization of a photonic qubit},

author = {A. Oliveira and R. Gomes and V. Brasil and N. R. Silva and L. Céleri and P. S. Ribeiro},

doi = {10.1016/j.physleta.2020.126933},

year = {2020},

date = {2020-12-01},

journal = {Physics Letters A},

volume = {384},

number = {36},

pages = {126933},

publisher = {Elsevier BV},

abstract = {The generalized amplitude damping (GAD) quantum channel implements the interaction between a qubit and an environment with arbitrary temperature and arbitrary interaction time. Here, we implement a photonic version of the GAD for the case of infinite interaction time (full thermalization). We also show that this quantum channel works as a thermal bath with controlled temperature.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Munuera-Javaloy, C.; Ban, Y.; Chen, X.; Casanova, J.

Robust Detection of High-Frequency Signals at the Nanoscale Journal Article

In: Physical Review Applied, vol. 14, no. 5, pp. 054054, 2020.

@article{MunueraJavaloy2020,

title = {Robust Detection of High-Frequency Signals at the Nanoscale},

author = {C. Munuera-Javaloy and Y. Ban and X. Chen and J. Casanova},

doi = {10.1103/physrevapplied.14.054054},

year = {2020},

date = {2020-11-01},

journal = {Physical Review Applied},

volume = {14},

number = {5},

pages = {054054},

publisher = {American Physical Society (APS)},

abstract = {We present a method relying on shortcuts to adiabaticity to achieve quantum detection of high-frequency signals at the nanoscale in a robust manner. More specifically, our protocol delivers tailored amplitudes and frequencies for control fields that, firstly, enable the coupling of the sensor with high-frequency signals and, secondly, minimize errors that would otherwise spoil the detection process. To exemplify the method, we particularize to detection of signals emitted by fast-rotating nuclear spins with nitrogen-vacancy-center quantum sensors. However, our protocol is straightforwardly applicable to other quantum devices such as silicon-vacancy centers, germanium-vacancy centers, or divacancies in silicon carbide.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Huang, T. -Y.; Zhang, J.; Li, J.; Chen, X.

Time-optimal variational control of a bright matter-wave soliton Journal Article

In: Physical Review A, vol. 102, no. 5, pp. 053313, 2020.

@article{Huang2020d,

title = {Time-optimal variational control of a bright matter-wave soliton},

author = {T. -Y. Huang and J. Zhang and J. Li and X. Chen},

doi = {10.1103/physreva.102.053313},

year = {2020},

date = {2020-11-01},

journal = {Physical Review A},

volume = {102},

number = {5},

pages = {053313},

publisher = {American Physical Society (APS)},

abstract = {Motivated by recent experiments, we present the time-optimal variational control of a bright matter-wave soliton trapped in the harmonic trap by manipulating the atomic interaction through Feshbach resonances. More specifically, we first apply the variational technique to derive the motion equation for capturing the soliton's shape and, second, combine an inverse-engineering method with optimal control theory to design the scatter length for implementing time-optimal decompression. Since the minimum-time solution is of the "bang-bang" type, the smooth regularization is further adopted to smooth the on-off controller out, thus avoiding the heating and atom loss induced from the magnetic field ramp across a Feshbach resonance, in practice.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Dassonneville, R.; Assouly, R.; Peronnin, T.; Rouchon, P.; Huard, B.

Number-Resolved Photocounter for Propagating Microwave Mode Journal Article

In: Physical Review Applied, vol. 14, iss. 4, pp. 044022, 2020.

@article{Dassonneville2020a,

title = {Number-Resolved Photocounter for Propagating Microwave Mode},

author = {R. Dassonneville and R. Assouly and T. Peronnin and P. Rouchon and B. Huard},

doi = {10.1103/PhysRevApplied.14.044022},

year = {2020},

date = {2020-10-01},

journal = {Physical Review Applied},

volume = {14},

issue = {4},

pages = {044022},

publisher = {American Physical Society},

abstract = {Detectors of propagating microwave photons have recently been realized using superconducting circuits. However, a number-resolved photocounter is still missing. In this article, we demonstrate a single-shot counter for propagating microwave photons that can resolve up to three photons. It is based on a pumped Josephson ring modulator that can catch an arbitrary propagating mode by frequency conversion and store its quantum state in a stationary memory mode. A transmon qubit then counts the number of photons in the memory mode using a series of binary questions. Using measurement-based feedback, the number of questions is minimal and scales logarithmically with the maximal number of photons. The detector features a detection efficiency of 0.96pm0.04, and a dark-count probability of 0.030pm0.002 for an average dead time of 4.5mu s. To maximize its performance, the device is first used as an in situ waveform detector from which an optimal pump is computed and applied. Depending on the number of incoming photons, the detector succeeds with a probability that ranges from 54pm2 to 99%.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Casariego, M.; Omar, Y.; Sanz, M.

Bi-frequency illumination: a quantum-enhanced protocol Miscellaneous

2020.

@misc{Casariego2020,

title = {Bi-frequency illumination: a quantum-enhanced protocol},

author = {M. Casariego and Y. Omar and M. Sanz},

url = {https://arxiv.org/abs/2010.15097},

year = {2020},

date = {2020-10-01},

abstract = {We propose a quantum-enhanced sensing protocol to measure the response of a target object to the frequency of a probe. In the model, a bi-frequency state illuminates a target, modeled as a beam splitter with a frequency-dependent reflectivity $eta(ømega)$, and embedded in a noisy thermal bath. After a lossy interaction with the object, we estimate $łambda = eta(ømega_2)-eta(ømega_1)$ for close enough frequencies $ømega_1$ and $ømega_2$. We show a quantum advantage by computing the quantum Fisher information, and proving that a two-mode squeezed state performs better than the classical state comprised of coherent states. This quantum advantage grows with the mean reflectivity of the probed object, and is noise-resilient. We derive explicit formulas for the optimal observables, proposing simple implementations with basic quantum optical transformations. We argue that our results can find applications in both radar an medical imaging, in particular in the microwave domain, and suggest continuations of the work involving continuous-variable frequency entanglement.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Gonzalez-Raya, T.; Sanz, M.

Coplanar Antenna Design for Microwave Entangled Signals Propagating in Open Air Miscellaneous

2020.

@misc{GonzalezRaya2020,

title = {Coplanar Antenna Design for Microwave Entangled Signals Propagating in Open Air},

author = {T. Gonzalez-Raya and M. Sanz},

url = {https://arxiv.org/abs/2009.03021},

year = {2020},

date = {2020-09-01},

abstract = {Open-air microwave quantum communication and metrology protocols must be able to transfer quantum resources from a fridge, where they are created, into an environment dominated by thermal noise. Indeed, the states that carry such quantum resources are generated in a cryostat at $T_textin simeq 10^-2 $~K and with $Z_textin = 50 , Ømega$ intrinsic impedance, and require an antenna-like device to transfer them into the open air, characterized by an intrinsic impedance of $Z_textout = 377 , Ømega$ and a temperature of $T_textout simeq 300$ K, with minimal losses. This device accomplishes a smooth impedance matching between the cryostat and the open air. Here, we study the transmission of two-mode squeezed thermal states, developing a technique to design the optimal shape of a coplanar antenna to preserve the entanglement. Based on a numerical optimization procedure we find the optimal shape of the impedance is exponential, and we adjust this shape to an analytical function. Additionally, this study reveals that losses are very sensitive to this shape, and small changes dramatically affect the outcoming entanglement, which could have been a limitation in previous experiments employing commercial antennae. This work will impact the fields of quantum sensing and quantum metrology, as well as any open-air microwave quantum communication protocol, with special application to the development of the quantum radar.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Kokkoniemi, R.; Girard, J. -P.; Hazra, D.; Laitinen, A.; Govenius, J.; Lake, R. E.; Sallinen, I.; Vesterinen, V.; Partanen, M.; Tan, J. Y.; Chan, K. W.; Tan, K. Y.; Hakonen, P.; Möttönen, M.

Bolometer operating at the threshold for circuit quantum electrodynamics Journal Article

In: Nature, vol. 586, no. 7827, pp. 47–51, 2020.

@article{Kokkoniemi2020,

title = {Bolometer operating at the threshold for circuit quantum electrodynamics},

author = {R. Kokkoniemi and J. -P. Girard and D. Hazra and A. Laitinen and J. Govenius and R. E. Lake and I. Sallinen and V. Vesterinen and M. Partanen and J. Y. Tan and K. W. Chan and K. Y. Tan and P. Hakonen and M. Möttönen},

doi = {10.1038/s41586-020-2753-3},

year = {2020},

date = {2020-09-01},

journal = {Nature},

volume = {586},

number = {7827},

pages = {47--51},

publisher = {Springer Science and Business Media LLC},

abstract = {Radiation sensors based on the heating effect of absorbed radiation are typically simple to operate and flexible in terms of input frequency, so they are widely used in gas detection1, security2, terahertz imaging3, astrophysical observations4 and medical applications5. Several important applications are currently emerging from quantum technology and especially from electrical circuits that behave quantum mechanically, that is, circuit quantum electrodynamics6. This field has given rise to single-photon microwave detectors7,8,9 and a quantum computer that is superior to classical supercomputers for certain tasks10. Thermal sensors hold potential for enhancing such devices because they do not add quantum noise and they are smaller, simpler and consume about six orders of magnitude less power than the frequently used travelling-wave parametric amplifiers11. However, despite great progress in the speed12 and noise levels13 of thermal sensors, no bolometer has previously met the threshold for circuit quantum electrodynamics, which lies at a time constant of a few hundred nanoseconds and a simultaneous energy resolution of the order of 10h gigahertz (where h is the Planck constant). Here we experimentally demonstrate a bolometer that operates at this threshold, with a noise-equivalent power of 30 zeptowatts per square-root hertz, comparable to the lowest value reported so far13, at a thermal time constant two orders of magnitude shorter, at 500 nanoseconds. Both of these values are measured directly on the same device, giving an accurate estimation of 30h gigahertz for the calorimetric energy resolution. These improvements stem from the use of a graphene monolayer with extremely low specific heat14 as the active material. The minimum observed time constant of 200 nanoseconds is well below the dephasing times of roughly 100 microseconds reported for superconducting qubits15 and matches the timescales of currently used readout schemes16,17, thus enabling circuit quantum electrodynamics applications for bolometers.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Goes, B. O.; Landi, G. T.; Solano, E.; Sanz, M.; Céleri, L. C.

Wehrl entropy production rate across a dynamical quantum phase transition Journal Article

In: Physical Review Research, vol. 2, no. 3, pp. 033419, 2020.

@article{Goes2020,

title = {Wehrl entropy production rate across a dynamical quantum phase transition},

author = {B. O. Goes and G. T. Landi and E. Solano and M. Sanz and L. C. Céleri},

doi = {10.1103/physrevresearch.2.033419},

year = {2020},

date = {2020-09-01},

journal = {Physical Review Research},

volume = {2},

number = {3},

pages = {033419},

publisher = {American Physical Society (APS)},

abstract = {The quench dynamics of many-body quantum systems may exhibit nonanalyticities in the Loschmidt echo, a phenomenon known as dynamical phase transition (DPT). Despite considerable research into the underlying mechanisms behind this phenomenon, several open questions still remain. Motivated by this, we put forth a detailed study of DPTs from the perspective of quantum phase space and entropy production, a key concept in thermodynamics. We focus on the Lipkin-Meshkov-Glick model and use spin-coherent states to construct the corresponding Husimi-Q quasiprobability distribution. The entropy of the Q function, known as Wehrl entropy, provides a measure of the coarse-grained dynamics of the system and, therefore, evolves nontrivially even for closed systems. We show that critical quenches lead to a quasimonotonic growth of the Wehrl entropy in time, combined with small oscillations. The former reflects the information scrambling characteristic of these transitions and serves as a measure of entropy production. On the other hand, the small oscillations imply negative entropy production rates and therefore signal the recurrences of the Loschmidt echo. Finally, we also study a Gaussification of the model based on a modified Holstein-Primakoff approximation. This allows us to identify the relative contribution of the low-energy sector to the emergence of DPTs. The results presented in this article are relevant not only from the dynamical quantum phase transition perspective but also for the field of quantum thermodynamics, since they point out that the Wehrl entropy can be used as a viable measure of entropy production.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Bañuls, M. C.; Blatt, R.; Catani, J.; Celi, A.; Cirac, J. I.; Dalmonte, M.; Fallani, L.; Jansen, K.; Lewenstein, M.; Montangero, S.; Muschik, C. A.; Reznik, B.; Rico, E.; Tagliacozzo, L.; Acoleyen, K. V.; Verstraete, F.; Wiese, U. -J.; Wingate, M.; Zakrzewski, J.; Zoller, P.

Simulating lattice gauge theories within quantum technologies Journal Article

In: The European Physical Journal D, vol. 74, no. 8, 2020.

@article{Banuls2020,

title = {Simulating lattice gauge theories within quantum technologies},

author = {M. C. Bañuls and R. Blatt and J. Catani and A. Celi and J. I. Cirac and M. Dalmonte and L. Fallani and K. Jansen and M. Lewenstein and S. Montangero and C. A. Muschik and B. Reznik and E. Rico and L. Tagliacozzo and K. V. Acoleyen and F. Verstraete and U. -J. Wiese and M. Wingate and J. Zakrzewski and P. Zoller},

doi = {10.1140/epjd/e2020-100571-8},

year = {2020},

date = {2020-08-01},

journal = {The European Physical Journal D},

volume = {74},

number = {8},

publisher = {Springer Science and Business Media LLC},

abstract = {Lattice gauge theories, which originated from particle physics in the context of Quantum Chromodynamics (QCD), provide an important intellectual stimulus to further develop quantum information technologies. While one long-term goal is the reliable quantum simulation of currently intractable aspects of QCD itself, lattice gauge theories also play an important role in condensed matter physics and in quantum information science. In this way, lattice gauge theories provide both motivation and a framework for interdisciplinary research towards the development of special purpose digital and analog quantum simulators, and ultimately of scalable universal quantum computers. In this manuscript, recent results and new tools from a quantum science approach to study lattice gauge theories are reviewed. Two new complementary approaches are discussed: first, tensor network methods are presented - a classical simulation approach - applied to the study of lattice gauge theories together with some results on Abelian and non-Abelian lattice gauge theories. Then, recent proposals for the implementation of lattice gauge theory quantum simulators in different quantum hardware are reported, e.g., trapped ions, Rydberg atoms, and superconducting circuits. Finally, the first proof-of-principle trapped ions experimental quantum simulations of the Schwinger model are reviewed.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Häffner, T.; Zanin, G. L.; Gomes, R. M.; Céleri, L. C.; Ribeiro, P. H. Souto

Remote preparation of single photon vortex thermal states Journal Article

In: The European Physical Journal Plus, vol. 135, no. 7, 2020.

@article{Haeffner2020,

title = {Remote preparation of single photon vortex thermal states},

author = {T. Häffner and G. L. Zanin and R. M. Gomes and L. C. Céleri and P. H. Souto Ribeiro},

doi = {10.1140/epjp/s13360-020-00609-z},

year = {2020},

date = {2020-07-01},

journal = {The European Physical Journal Plus},

volume = {135},

number = {7},

publisher = {Springer Science and Business Media LLC},

abstract = {Photon pairs produced in spontaneous parametric down-conversion are naturally entangled in their transverse spatial degrees of freedom including the orbital angular momentum. Pumping a nonlinear crystal with a zero-order Gaussian mode produces quantum correlated signal and idler photons with equal orbital angular momentum and opposite signs. Measurements performed on one of the photons prepares the state of the other remotely. We study the remote state preparation in this system from the perspective of its potential application to Quantum Thermodynamics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Galicia, A.; Ramon, B.; Solano, E.; Sanz, M.

Enhanced connectivity of quantum hardware with digital-analog control Journal Article

In: Physical Review Research, vol. 2, no. 3, pp. 033103, 2020.

@article{Galicia2020,

title = {Enhanced connectivity of quantum hardware with digital-analog control},

author = {A. Galicia and B. Ramon and E. Solano and M. Sanz},

doi = {10.1103/physrevresearch.2.033103},

year = {2020},

date = {2020-07-01},

journal = {Physical Review Research},

volume = {2},

number = {3},

pages = {033103},

publisher = {American Physical Society (APS)},

abstract = {Quantum computers based on superconducting circuits are experiencing rapid development, with the aim to outperform classical computers in certain useful tasks in the near future. However, the currently available chip fabrication technologies limit the capability of gathering a large number of high-quality qubits in a single superconducting chip, a requirement for implementing quantum error correction. Furthermore, achieving high connectivity in a chip poses a formidable technological challenge. Here, we propose a hybrid digital-analog quantum algorithm that enhances the physical connectivity among qubits coupled by an arbitrary inhomogeneous nearest-neighbor Ising Hamiltonian and generates an arbitrary all-to-all Ising Hamiltonian only by employing single-qubit rotations. Additionally, we optimize the proposed algorithm in the number of analog blocks and in the time required for the simulation. These results take advantage of the natural evolution of the system by combining the flexibility of digital steps with the robustness of analog quantum computing, allowing us to improve the connectivity of the hardware and the efficiency of quantum algorithms.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Zhang, J.; Li, T.; Kokkoniemi, R.; Yan, C.; Liu, W.; Partanen, M.; Tan, K. Y.; He, M.; Ji, L.; Grönberg, L.; Möttönen, M.

Broadband tunable phase shifter for microwaves Journal Article

In: AIP Advances, vol. 10, no. 6, pp. 065128, 2020.

@article{Zhang2020,

title = {Broadband tunable phase shifter for microwaves},

author = {J. Zhang and T. Li and R. Kokkoniemi and C. Yan and W. Liu and M. Partanen and K. Y. Tan and M. He and L. Ji and L. Grönberg and M. Möttönen},

doi = {10.1063/5.0006499},

year = {2020},

date = {2020-06-01},

journal = {AIP Advances},

volume = {10},

number = {6},

pages = {065128},

publisher = {AIP Publishing},

abstract = {We implement a broadly tunable phase shifter for microwaves based on superconducting quantum interference devices (SQUIDs) and study it both experimentally and theoretically. At different frequencies, a unit transmission coefficient, |S21| = 1, can be theoretically achieved along a curve where the phase shift is controllable by the magnetic flux. The fabricated device consists of three equidistant SQUIDs interrupting a transmission line. We model each SQUID embedded at different positions along the transmission line with two parameters, capacitance and inductance, the values of which we extract from the experiments. In our experiments, the tunability of the phase shift varies from 0.07 x pi to 0.14 x pi radians along the full-transmission curve with the input frequency ranging from 6.00 GHz to 6.28 GHz. The reported measurements are in good agreement with simulations, which is promising for future design work of phase shifters for different applications.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Ding, Y.; Huang, T. -Y.; Paul, K.; Hao, M.; Chen, X.

Smooth bang-bang shortcuts to adiabaticity for atomic transport in a moving harmonic trap Journal Article

In: Physical Review A, vol. 101, no. 6, pp. 063410, 2020.

@article{Ding2020a,

title = {Smooth bang-bang shortcuts to adiabaticity for atomic transport in a moving harmonic trap},

author = {Y. Ding and T. -Y. Huang and K. Paul and M. Hao and X. Chen},

doi = {10.1103/physreva.101.063410},

year = {2020},

date = {2020-06-01},

journal = {Physical Review A},

volume = {101},

number = {6},

pages = {063410},

publisher = {American Physical Society (APS)},

abstract = {Bang-bang control is often used to implement a minimal-time shortcut to adiabaticity for efficient transport of atoms in a moving harmonic trap. However, drastic changes of the on-off controller, leading to high transport-mode excitation and energy consumption, become infeasible under realistic experimental conditions. To circumvent these problems, we propose smooth bang-bang protocols with near-minimal time, by setting the physical constraints on the relative displacement, speed, and acceleration between the mass center of the atom and the trap center. We adopt Pontryagin's maximum principle to obtain the analytical solutions of smooth bang-bang protocol for near-time-minimal control. More importantly, it is found that the energy excitation and sloshing amplitude are significantly reduced at the expense of operation time. We also present a multiple shooting method for the self-consistent numerical analysis. Finally, this method is applied to other tasks, e.g., energy minimization, where obtaining smooth analytical form is complicated.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Albarrán-Arriagada, F.; Retamal, J. C.; Solano, E.; Lamata, L.

Reinforcement learning for semi-autonomous approximate quantum eigensolver Journal Article

In: Machine Learning: Science and Technology, vol. 1, no. 1, pp. 015002, 2020.

@article{AlbarranArriagada2020,

title = {Reinforcement learning for semi-autonomous approximate quantum eigensolver},

author = {F. Albarrán-Arriagada and J. C. Retamal and E. Solano and L. Lamata},

doi = {10.1088/2632-2153/ab43b4},

year = {2020},

date = {2020-06-01},

journal = {Machine Learning: Science and Technology},

volume = {1},

number = {1},

pages = {015002},

publisher = {IOP Publishing},

abstract = {The characterization of an operator by its eigenvectors and eigenvalues allows us to know its action over any quantum state. Here, we propose a protocol to obtain an approximation of the eigenvectors of an arbitrary Hermitian quantum operator. This protocol is based on measurement and feedback processes, which characterize a reinforcement learning protocol. Our proposal is composed of two systems, a black box named environment and a quantum state named agent. The role of the environment is to change any quantum state by a unitary matrix $hatU_E=e^-itauhatmathcalO_E$ where $hatmathcalO_E$ is a Hermitian operator, and $tau$ is a real parameter. The agent is a quantum state which adapts to some eigenvector of $hatmathcalO_E$ by repeated interactions with the environment, feedback process, and semi-random rotations. With this proposal, we can obtain an approximation of the eigenvectors of a random qubit operator with average fidelity over 90% in less than 10 iterations, and surpass 98% in less than 300 iterations. Moreover, for the two-qubit cases, the four eigenvectors are obtained with fidelities above 89% in 8000 iterations for a random operator, and fidelities of $99%$ for an operator with the Bell states as eigenvectors. This protocol can be useful to implement semi-autonomous quantum devices which should be capable of extracting information and deciding with minimal resources and without human intervention.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Peronnin, T.; Marković, D.; Ficheux, Q.; Huard, B.

Sequential Dispersive Measurement of a Superconducting Qubit Journal Article

In: Physical Review Letters, vol. 124, no. 18, pp. 180502, 2020, ISSN: 0031-9007.

@article{Peronnin2020,

title = {Sequential Dispersive Measurement of a Superconducting Qubit},

author = {T. Peronnin and D. Marković and Q. Ficheux and B. Huard},

url = {https://link.aps.org/doi/10.1103/PhysRevLett.124.180502},

doi = {10.1103/PhysRevLett.124.180502},

issn = {0031-9007},

year = {2020},

date = {2020-05-01},

journal = {Physical Review Letters},

volume = {124},

number = {18},

pages = {180502},

publisher = {American Physical Society},

abstract = {We present a superconducting device that realizes the sequential measurement of a transmon qubit. The device disables common limitations of dispersive readout such as Purcell effect or transients in the cavity mode by turning on and off the coupling to the measurement channel on demand. The qubit measurement begins by loading a readout resonator that is coupled to the qubit. After an optimal interaction time with negligible loss, a microwave pump releases the content of the readout mode by upconversion into a measurement line in a characteristic time as low as 10 ns, which is 400 times shorter than the lifetime of the readout resonator. A direct measurement of the released field quadratures demonstrates a readout fidelity of 97.5% in a total measurement time of 220 ns. The Wigner tomography of the readout mode allows us to characterize the non-Gaussian nature of the readout mode and its dynamics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Olivares-Sánchez, J.; Casanova, J.; Solano, E.; Lamata, L.

Measurement-Based Adaptation Protocol with Quantum Reinforcement Learning in a Rigetti Quantum Computer Journal Article

In: Quantum Reports, vol. 2, no. 2, pp. 293–304, 2020.

@article{OlivaresSanchez2020,

title = {Measurement-Based Adaptation Protocol with Quantum Reinforcement Learning in a Rigetti Quantum Computer},

author = {J. Olivares-Sánchez and J. Casanova and E. Solano and L. Lamata},

doi = {10.3390/quantum2020019},

year = {2020},

date = {2020-05-01},

journal = {Quantum Reports},

volume = {2},

number = {2},

pages = {293--304},

publisher = {MDPI AG},

abstract = {We present an experimental realisation of a measurement-based adaptation protocol with quantum reinforcement learning in a Rigetti cloud quantum computer. The experiment in this few-qubit superconducting chip faithfully reproduces the theoretical proposal, setting the first steps towards a semiautonomous quantum agent. This experiment paves the way towards quantum reinforcement learning with superconducting circuits.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Huang, T. -Y.; Malomed, B. A.; Chen, X.

Shortcuts to adiabaticity for an interacting Bose-Einstein condensate via exact solutions of the generalized Ermakov equation Journal Article

In: Chaos: An Interdisciplinary Journal of Nonlinear Science, vol. 30, no. 5, pp. 053131, 2020.

@article{Huang2020b,

title = {Shortcuts to adiabaticity for an interacting Bose-Einstein condensate via exact solutions of the generalized Ermakov equation},

author = {T. -Y. Huang and B. A. Malomed and X. Chen},

doi = {10.1063/5.0004309},

year = {2020},

date = {2020-05-01},

journal = {Chaos: An Interdisciplinary Journal of Nonlinear Science},

volume = {30},

number = {5},

pages = {053131},

publisher = {AIP Publishing},

abstract = {Shortcuts to adiabatic expansion of the effectively one-dimensional Bose-Einstein condensate (BEC) loaded in the harmonic-oscillator (HO) trap are investigated by combining techniques of variational approximation and inverse engineering. Piecewise-constant (discontinuous) intermediate trap frequencies, similar to the known bang-bang forms in the optimal-control theory, are derived from an exact solution of a generalized Ermakov equation. Control schemes considered in the paper include imaginary trap frequencies at short time scales, i.e., the HO potential replaced by the quadratic repulsive one. Taking into regard the BEC's intrinsic nonlinearity, results are reported for the minimal transfer time, excitation energy (which measures deviation from the effective adiabaticity), and stability for the shortcut-to-adiabaticity protocols. These results are not only useful for the realization of fast frictionless cooling, but also help us to address fundamental problems of the quantum speed limit and thermodynamics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Wang, Z.; Casanova, J.; Plenio, M. B.

Enhancing the Robustness of Dynamical Decoupling Sequences with Correlated Random Phases Journal Article

In: Symmetry, vol. 12, no. 5, pp. 730, 2020.

@article{Wang2020a,

title = {Enhancing the Robustness of Dynamical Decoupling Sequences with Correlated Random Phases},

author = {Z. Wang and J. Casanova and M. B. Plenio},

doi = {10.3390/sym12050730},

year = {2020},

date = {2020-05-01},

journal = {Symmetry},

volume = {12},

number = {5},

pages = {730},

publisher = {MDPI AG},

abstract = {We show that the addition of correlated phases to the recently developed method of randomized dynamical decoupling pulse sequences can improve its performance in quantum sensing. In particular, by correlating the relative phases of basic pulse units in dynamical decoupling sequences, we are able to improve the suppression of the signal distortion due to pi pulse imperfections and spurious responses due to finite-width pi pulses. This enhances the selectivity of quantum sensors such as those based on NV centers in diamond.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Hu, F.; Lamata, L.; Wang, C.; Chen, X.; Solano, E.; Sanz, M.

Quantum Advantage in Cryptography with a Low-Connectivity Quantum Annealer Journal Article

In: Physical Review Applied, vol. 13, no. 5, pp. 054062, 2020.

@article{Hu2020a,

title = {Quantum Advantage in Cryptography with a Low-Connectivity Quantum Annealer},

author = {F. Hu and L. Lamata and C. Wang and X. Chen and E. Solano and M. Sanz},

doi = {10.1103/physrevapplied.13.054062},

year = {2020},

date = {2020-05-01},

journal = {Physical Review Applied},

volume = {13},

number = {5},

pages = {054062},

publisher = {American Physical Society (APS)},

abstract = {The application in cryptography of quantum algorithms for prime factorization fostered the interest in quantum computing. However, quantum computers, and particularly quantum annealers, can also be helpful to construct secure cryptographic keys. Indeed, finding robust Boolean functions for cryptography is an important problem in sequence ciphers, block ciphers, and hash functions, among others. Due to the superexponential size O(22n) of the associated space, finding n-variable Boolean functions with global cryptographic constraints is computationally hard. This problem has already been addressed employing generic low-connected incoherent D-Wave quantum annealers. However, the limited connectivity of the Chimera graph, together with the exponential growth in the complexity of the Boolean-function design problem, limit the problem scalability. Here, we propose a special-purpose coherent quantum-annealing architecture with three couplers per qubit, designed to optimally encode the bent-function design problem. A coherent quantum annealer with this tree-type architecture has the potential to solve the eight-variable bent-function design problem, which is classically unsolved, with only 127 physical qubits and 126 couplers. This paves the way to reach useful quantum supremacy within the framework of quantum annealing for cryptographic purposes.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Ribeiro, P. H. S.; Häffner, T.; Zanin, G. L.; Silva, N. R.; Araújo, R. M.; Soares, W. C.; Assis, R. J.; Céleri, L. C.; Forbes, A.

Experimental study of the generalized Jarzynski fluctuation relation using entangled photons Journal Article

In: Physical Review A, vol. 101, no. 5, pp. 052113, 2020.

@article{Ribeiro2020,

title = {Experimental study of the generalized Jarzynski fluctuation relation using entangled photons},

author = {P. H. S. Ribeiro and T. Häffner and G. L. Zanin and N. R. Silva and R. M. Araújo and W. C. Soares and R. J. Assis and L. C. Céleri and A. Forbes},

doi = {10.1103/physreva.101.052113},

year = {2020},

date = {2020-05-01},

journal = {Physical Review A},

volume = {101},

number = {5},

pages = {052113},

publisher = {American Physical Society (APS)},

abstract = {Optical modes possessing orbital angular momentum constitute a very useful platform for experimental studies on the quantum limits of thermodynamics. Here, we present experimental results for entangled photon pairs subjected to thin turbulence simulated with spatial light modulators and interpret them in the context of the generalized Jarzynski's fluctuation relation. By holographic measurement of the orbital angular momentum, we obtain the work distribution produced by the turbulence for single- and double-sided turbulence channels. The use of Klyshko's advanced-wave picture allows us to interpret the experimental scheme as a two-way process in a fully quantum picture.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Hu, F.; Lamata, L.; Sanz, M.; Chen, X.; Chen, X.; Wang, C.; Solano, E.

Quantum computing cryptography: Finding cryptographic Boolean functions with quantum annealing by a 2000 qubit D-wave quantum computer Journal Article

In: Physics Letters, Section A: General, Atomic and Solid State Physics, vol. 384, no. 10, pp. 126214, 2020, ISSN: 0375-9601.

@article{Hu2020,

title = {Quantum computing cryptography: Finding cryptographic Boolean functions with quantum annealing by a 2000 qubit D-wave quantum computer},

author = {F. Hu and L. Lamata and M. Sanz and X. Chen and X. Chen and C. Wang and E. Solano},

doi = {10.1016/j.physleta.2019.126214},

issn = {0375-9601},

year = {2020},

date = {2020-04-01},

journal = {Physics Letters, Section A: General, Atomic and Solid State Physics},

volume = {384},

number = {10},

pages = {126214},

publisher = {Elsevier B.V.},

abstract = {As the building block in symmetric cryptography, designing Boolean functions satisfying multiple properties is an important problem in sequence ciphers, block ciphers, and hash functions. However, the search of n-variable Boolean functions fulfilling global cryptographic constraints is computationally hard due to the super-exponential size O(22n) of the space. Here, we introduce a codification of the cryptographically relevant constraints in the ground state of an Ising Hamiltonian, allowing us to naturally encode it in a quantum annealer, which seems to provide a quantum speedup. Additionally, we benchmark small n cases in a D-Wave machine, showing its capacity of devising cryptographic Boolean functions with certain relevant properties. We have complemented it with local search and chain repair to improve the D-Wave quantum annealer performance related to the low connectivity. This work shows how to codify super-exponential cryptographic problems into quantum annealers and paves the way for reaching quantum supremacy.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Ding, Y.; Martín-Guerrero, J. D.; Sanz, M.; Magdalena-Benedicto, R.; Chen, X.; Solano, E.

Retrieving Quantum Information with Active Learning Journal Article

In: Physical Review Letters, vol. 124, no. 14, pp. 140504, 2020, ISSN: 0031-9007.

@article{Ding2020,

title = {Retrieving Quantum Information with Active Learning},

author = {Y. Ding and J. D. Martín-Guerrero and M. Sanz and R. Magdalena-Benedicto and X. Chen and E. Solano},

url = {https://link.aps.org/doi/10.1103/PhysRevLett.124.140504},

doi = {10.1103/PhysRevLett.124.140504},

issn = {0031-9007},

year = {2020},

date = {2020-04-01},

journal = {Physical Review Letters},

volume = {124},

number = {14},

pages = {140504},

publisher = {American Physical Society (APS)},

abstract = {Active learning is a machine learning method aiming at optimal design for model training. At variance with supervised learning, which labels all samples, active learning provides an improved model by labeling samples with maximal uncertainty according to the estimation model. Here, we propose the use of active learning for efficient quantum information retrieval, which is a crucial task in the design of quantum experiments. Meanwhile, when dealing with large data output, we employ active learning for the sake of classification with minimal cost in fidelity loss. Indeed, labeling only 5% samples, we achieve almost 90% rate estimation. The introduction of active learning methods in the data analysis of quantum experiments will enhance applications of quantum technologies.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Cong, L.; Felicetti, S.; Casanova, J.; Lamata, L.; Solano, E.; Arrazola, I.

Selective interactions in the quantum Rabi model Journal Article

In: Physical Review A, vol. 101, no. 3, pp. 032350, 2020, ISSN: 2469-9926.

@article{Cong2020,

title = {Selective interactions in the quantum Rabi model},

author = {L. Cong and S. Felicetti and J. Casanova and L. Lamata and E. Solano and I. Arrazola},

url = {https://link.aps.org/doi/10.1103/PhysRevA.101.032350},

doi = {10.1103/PhysRevA.101.032350},

issn = {2469-9926},

year = {2020},

date = {2020-03-01},

journal = {Physical Review A},

volume = {101},

number = {3},

pages = {032350},

publisher = {American Physical Society},

abstract = {We demonstrate the emergence of selective k-photon interactions in the strong and ultrastrong coupling regimes of the quantum Rabi model with a Stark coupling term. In particular, we show that the interplay between the rotating and counterrotating terms produces multiphoton interactions whose resonance frequencies depend, due to the Stark term, on the state of the bosonic mode. We develop an analytical framework to explain these k-photon interactions by using time-dependent perturbation theory. Finally, we propose a method to achieve the quantum simulation of the quantum Rabi model with a Stark term by using the internal and vibrational degrees of freedom of a trapped ion, and demonstrate its performance with numerical simulations considering realistic physical parameters.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Capela, M.; Céleri, L. C.; Modi, K.; Chaves, R.

Monogamy of temporal correlations: Witnessing non-Markovianity beyond data processing Journal Article

In: Physical Review Research, vol. 2, no. 1, pp. 013350, 2020.

@article{Capela2020,

title = {Monogamy of temporal correlations: Witnessing non-Markovianity beyond data processing},

author = {M. Capela and L. C. Céleri and K. Modi and R. Chaves},

doi = {10.1103/physrevresearch.2.013350},

year = {2020},

date = {2020-03-01},

journal = {Physical Review Research},

volume = {2},

number = {1},

pages = {013350},

publisher = {American Physical Society (APS)},

abstract = {The modeling of natural phenomena via a Markov process --- a process for which the future is independent of the past, given the present--- is ubiquitous in many fields of science. Within this context, it is of foremost importance to develop ways to check from the available empirical data if the underlying mechanism is indeed Markovian. A paradigmatic example is given by data processing inequalities, the violation of which is an unambiguous proof of the non-Markovianity of the process. Here, our aim is twofold. First we show the existence of a monogamy-like type of constraints, beyond data processing, respected by Markov chains. Second, to show a novel connection between the quantification of causality and the violation of both data processing and monogamy inequalities. Apart from its foundational relevance in the study of stochastic processes we also consider the applicability of our results in a typical quantum information setup, showing it can be useful to witness the non-Markovianity arising in a sequence of quantum non-projective measurements.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Munuera-Javaloy, C.; Arrazola, I.; Solano, E.; Casanova, J.

Double quantum magnetometry at large static magnetic fields Journal Article

In: Physical Review B, vol. 101, no. 10, pp. 104411, 2020, ISSN: 2469-9950.

@article{MunueraJavaloy2020a,

title = {Double quantum magnetometry at large static magnetic fields},

author = {C. Munuera-Javaloy and I. Arrazola and E. Solano and J. Casanova},

url = {https://link.aps.org/doi/10.1103/PhysRevB.101.104411},

doi = {10.1103/PhysRevB.101.104411},

issn = {2469-9950},

year = {2020},

date = {2020-03-01},

journal = {Physical Review B},

volume = {101},

number = {10},

pages = {104411},

publisher = {American Physical Society},

abstract = {We present a protocol to achieve double quantum magnetometry at large static magnetic fields. This is a regime where sensitive sample parameters, such as the chemical shift, get enhanced facilitating their characterization. In particular, our method delivers two-Tone stroboscopic radiation patterns with modulated Rabi frequencies to achieve larger spectral signals. Furthermore, it does not introduce inhomogeneous broadening in the sample spectrum preventing signal misinterpretation. Moreover, our protocol is designed to work under realistic conditions such as the presence of moderate microwave power and errors on the radiation fields. Albeit we particularize to nitrogen vacancy centers, our protocol is general, thus applicable to distinct quantum sensors.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Parra-Rodriguez, A.; Lougovski, P.; Lamata, L.; Solano, E.; Sanz, M.

Digital-analog quantum computation Journal Article

In: Physical Review A, vol. 101, no. 2, pp. 022305, 2020, ISSN: 2469-9926.

@article{ParraRodriguez2020a,

title = {Digital-analog quantum computation},

author = {A. Parra-Rodriguez and P. Lougovski and L. Lamata and E. Solano and M. Sanz},

url = {https://link.aps.org/doi/10.1103/PhysRevA.101.022305},

doi = {10.1103/PhysRevA.101.022305},

issn = {2469-9926},

year = {2020},

date = {2020-02-01},

journal = {Physical Review A},

volume = {101},

number = {2},

pages = {022305},

abstract = {Digital quantum computing paradigm offers highly desirable features such as universality, scalability, and quantum error correction. However, physical resource requirements to implement useful error-corrected quantum algorithms are prohibitive in the current era of NISQ devices. As an alternative path to performing universal quantum computation, within the NISQ era limitations, we propose to merge digital single-qubit operations with analog multiqubit entangling blocks in an approach we call digital-analog quantum computing (DAQC). Along these lines, although the techniques may be extended to any resource, we propose to use unitaries generated by the ubiquitous Ising Hamiltonian for the analog entangling block and we prove its universal character. We construct explicit DAQC protocols for efficient simulations of arbitrary inhomogeneous Ising, two-body, and M-body spin Hamiltonian dynamics by means of single-qubit gates and a fixed homogeneous Ising Hamiltonian. Additionally, we compare a sequential approach where the interactions are switched on and off (stepwise DAQC) with an always-on multiqubit interaction interspersed by fast single-qubit pulses (banged DAQC). Finally, we perform numerical tests comparing purely digital schemes with DAQC protocols, showing a remarkably better performance of the latter. The proposed DAQC approach combines the robustness of analog quantum computing with the flexibility of digital methods.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Gonzalez-Raya, T.; Lukens, J. M.; Céleri, L. C.; Sanz, M.

Quantum Memristors in Frequency-Entangled Optical Fields Journal Article

In: Materials, vol. 13, no. 4, pp. 864, 2020, ISSN: 1996-1944.

@article{GonzalezRaya2020a,

title = {Quantum Memristors in Frequency-Entangled Optical Fields},

author = {T. Gonzalez-Raya and J. M. Lukens and L. C. Céleri and M. Sanz},

url = {https://www.mdpi.com/1996-1944/13/4/864},

doi = {10.3390/ma13040864},

issn = {1996-1944},

year = {2020},

date = {2020-02-01},

journal = {Materials},

volume = {13},

number = {4},

pages = {864},

publisher = {Multidisciplinary Digital Publishing Institute},

abstract = {A quantum memristor is a passive resistive circuit element with memory, engineered in a given quantum platform. It can be represented by a quantum system coupled to a dissipative environment, in which a system-bath coupling is mediated through a weak measurement scheme and classical feedback on the system. In quantum photonics, such a device can be designed from a beam splitter with tunable reflectivity, which is modified depending on the results of measurements in one of the outgoing beams. Here, we show that a similar implementation can be achieved with frequency-entangled optical fields and a frequency mixer that, working similarly to a beam splitter, produces state superpositions. We show that the characteristic hysteretic behavior of memristors can be reproduced when analyzing the response of the system with respect to the control, for different experimentally attainable states. Since memory effects in memristors can be exploited for classical and neuromorphic computation, the results presented in this work could be a building block for constructing quantum neural networks in quantum photonics, when scaling up.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Silva, F.; Sanz, M.; Seixas, J.; Solano, E.; Omar, Y.

Perceptrons from memristors Journal Article

In: Neural Networks, vol. 122, pp. 273–278, 2020, ISSN: 1879-2782.

@article{Silva2020,

title = {Perceptrons from memristors},

author = {F. Silva and M. Sanz and J. Seixas and E. Solano and Y. Omar},

doi = {10.1016/j.neunet.2019.10.013},

issn = {1879-2782},

year = {2020},

date = {2020-02-01},

journal = {Neural Networks},

volume = {122},

pages = {273--278},

publisher = {Elsevier Ltd},

abstract = {Memristors, resistors with memory whose outputs depend on the history of their inputs, have been used with success in neuromorphic architectures, particularly as synapses and non-volatile memories. However, to the best of our knowledge, no model for a network in which both the synapses and the neurons are implemented using memristors has been proposed so far. In the present work we introduce models for single and multilayer perceptrons based exclusively on memristors. We adapt the delta rule to the memristor-based single-layer perceptron and the backpropagation algorithm to the memristor-based multilayer perceptron. Our results show that both perform as expected for perceptrons, including satisfying Minsky-Papert's theorem. As a consequence of the Universal Approximation Theorem, they also show that memristors are universal function approximators. By using memristors for both the neurons and the synapses, our models pave the way for novel memristor-based neural network architectures and algorithms. A neural network based on memristors could show advantages in terms of energy conservation and open up possibilities for other learning systems to be adapted to a memristor-based paradigm, both in the classical and quantum learning realms.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Headley, D.; Müller, T.; Martin, A.; Solano, E.; Sanz, M.; Wilhelm, F. K.

Approximating the Quantum Approximate Optimisation Algorithm Miscellaneous

2020.

@misc{Headley2020,

title = {Approximating the Quantum Approximate Optimisation Algorithm},

author = {D. Headley and T. Müller and A. Martin and E. Solano and M. Sanz and F. K. Wilhelm},

url = {http://arxiv.org/abs/2002.12215},

year = {2020},

date = {2020-02-01},

abstract = {The Quantum Approximate Optimisation Algorithm was proposed as a heuristic method for solving combinatorial optimisation problems on near-term quantum computers and may be among the first algorithms to perform useful computations in the post-supremacy, noisy, intermediate scale era of quantum computing. In this work, we exploit the recently proposed digital-analog quantum computation paradigm, in which the versatility of programmable universal quantum computers and the error resilience of quantum simulators are combined to improve platforms for quantum computation. We show that the digital-analog paradigm is suited to the variational quantum approximate optimisation algorithm, due to its inherent resilience against coherent errors, by performing large-scale simulations and providing analytical bounds for its performance in devices with finite single-qubit operation times. We observe regimes of single-qubit operation speed in which the considered variational algorithm provides a significant improvement over non-variational counterparts.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Osada, T.; Coutinho, B.; Omar, Y.; Sanaka, K.; Munro, W. J.; Nemoto, K.

Continuous-time quantum-walk spatial search on the Bollobás scale-free network Journal Article

In: Physical Review A, vol. 101, no. 2, pp. 022310, 2020, ISSN: 2469-9934.

@article{Osada2020,

title = {Continuous-time quantum-walk spatial search on the Bollobás scale-free network},

author = {T. Osada and B. Coutinho and Y. Omar and K. Sanaka and W. J. Munro and K. Nemoto},

doi = {10.1103/PhysRevA.101.022310},

issn = {2469-9934},

year = {2020},

date = {2020-02-01},

journal = {Physical Review A},

volume = {101},

number = {2},

pages = {022310},

publisher = {American Physical Society},

abstract = {The scale-free property emerges in various real-world networks and is an essential property that characterizes the dynamics or features of such networks. In this work, we investigate the effect of this scale-free property on a quantum information processing task of finding a marked node in the network, known as the quantum spatial search. We analyze the quantum spatial search algorithm using a continuous-time quantum walk on the Bollobás network, and we evaluate the time T to localize the quantum walker on the marked node starting from an unbiased initial state. Our main finding is that T is determined by the global structure around the marked node, while some local information of the marked node, such as the degree, does not identify T. We discuss this by examining the correlation between T and some centrality measures of the network, and we show that the closeness centrality of the marked node is highly correlated with T. We also characterize the distribution of T by marking different nodes in the network, which displays a multimode log-normal distribution. Especially on the Bollobás network, T is a few orders of magnitude shorter depending on whether the marked node is adjacent to the largest degree hub node. However, as T depends on the property of the marked node, one requires some amount of prior knowledge about such a property of the marked node in order to identify the optimal time to measure the quantum walker and achieve a fast search. These results indicate that the existence of the hub node in the scale-free network plays a crucial role in the quantum spatial search.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Arrazola, I.; Plenio, M. B.; Solano, E.; Casanova, J.

Hybrid Microwave-Radiation Patterns for High-Fidelity Quantum Gates with Trapped Ions Journal Article

In: Physical Review Applied, vol. 13, no. 2, pp. 024068, 2020, ISSN: 2331-7019.

@article{Arrazola2020,

title = {Hybrid Microwave-Radiation Patterns for High-Fidelity Quantum Gates with Trapped Ions},

author = {I. Arrazola and M. B. Plenio and E. Solano and J. Casanova},

url = {https://link.aps.org/doi/10.1103/PhysRevApplied.13.024068},

doi = {10.1103/PhysRevApplied.13.024068},

issn = {2331-7019},

year = {2020},

date = {2020-02-01},

journal = {Physical Review Applied},

volume = {13},

number = {2},

pages = {024068},

publisher = {American Physical Society},

abstract = {We present a method that combines continuous and pulsed microwave-radiation patterns to achieve robust interactions among hyperfine trapped ions placed in a magnetic field gradient. More specifically, our scheme displays continuous microwave drivings with modulated phases, phase flips, and $pi$ pulses. This leads to high-fidelity entangling gates that are resilient against magnetic field fluctuations, changes in the microwave amplitudes, and crosstalk effects. Our protocol runs with arbitrary values of microwave power, which includes the technologically relevant case of low microwave intensities. We demonstrate the performance of our method with detailed numerical simulations that take into account the main sources of decoherence.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Gonzalez-Raya, T.; Solano, E.; Sanz, M.

Quantized Three-Ion-Channel Neuron Model for Neural Action Potentials Journal Article

In: Quantum, vol. 4, pp. 224, 2020, ISSN: 2521-327X.

@article{GonzalezRaya2020b,

title = {Quantized Three-Ion-Channel Neuron Model for Neural Action Potentials},

author = {T. Gonzalez-Raya and E. Solano and M. Sanz},

url = {https://quantum-journal.org/papers/q-2020-01-20-224/},

doi = {10.22331/q-2020-01-20-224},

issn = {2521-327X},

year = {2020},

date = {2020-01-01},

journal = {Quantum},

volume = {4},

pages = {224},

publisher = {Verein zur Forderung des Open Access Publizierens in den Quantenwissenschaften},

abstract = {The Hodgkin-Huxley model describes the conduction of the nervous impulse through the axon, whose membrane's electric response can be described employing multiple connected electric circuits containing capacitors, voltage sources, and conductances. These conductances depend on previous depolarizing membrane voltages, which can be identified with a memory resistive element called memristor. Inspired by the recent quantization of the memristor, a simplified Hodgkin-Huxley model including a single ion channel has been studied in the quantum regime. Here, we study the quantization of the complete Hodgkin-Huxley model, accounting for all three ion channels, and introduce a quantum source, together with an output waveguide as the connection to a subsequent neuron. Our system consists of two memristors and one resistor, describing potassium, sodium, and chloride ion channel conductances, respectively, and a capacitor to account for the axon's membrane capacitance. We study the behavior of both ion channel conductivities and the circuit voltage, and we compare the results with those of the single channel, for a given quantum state of the source. It is remarkable that, in opposition to the single-channel model, we are able to reproduce the voltage spike in an adiabatic regime. Arguing that the circuit voltage is a quantum variable, we find a purely quantum-mechanical contribution in the system voltage's second moment. This work represents a complete study of the Hodgkin-Huxley model in the quantum regime, establishing a recipe for constructing quantum neuron networks with quantum state inputs. This paves the way for advances in hardware-based neuromorphic quantum computing, as well as quantum machine learning, which might be more efficient resource-wise.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Martin, A.; Lamata, L.; Solano, E.; Sanz, M.

Digital-analog quantum algorithm for the quantum Fourier transform Journal Article

In: Physical Review Research, vol. 2, no. 1, pp. 013012, 2020.

@article{Martin2020,

title = {Digital-analog quantum algorithm for the quantum Fourier transform},

author = {A. Martin and L. Lamata and E. Solano and M. Sanz},

doi = {10.1103/physrevresearch.2.013012},

year = {2020},

date = {2020-01-01},

journal = {Physical Review Research},

volume = {2},

number = {1},

pages = {013012},

publisher = {American Physical Society (APS)},

abstract = {Quantum computers will allow calculations beyond existing classical computers. However, current technology is still too noisy and imperfect to construct a universal digital quantum computer with quantum error correction. Inspired by the evolution of classical computation, an alternative paradigm merging the flexibility of digital quantum computation with the robustness of analog quantum simulation has emerged. This universal paradigm is known as digital-analog quantum computing. Here, we introduce an efficient digital-analog quantum algorithm to compute the quantum Fourier transform, a subroutine widely employed in several relevant quantum algorithms. We show that, under reasonable assumptions about noise models, the fidelity of the quantum Fourier transformation improves considerably using this approach when the number of qubits involved grows. This suggests that, in the Noisy Intermediate-Scale Quantum (NISQ) era, hybrid protocols combining digital and analog quantum computing could be a sensible approach to reach useful quantum supremacy.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Borim, D.; Céleri, L. C.; Kiosses, V. I.

Precision in estimating Unruh temperature Miscellaneous

2020.

@misc{Borim2020,

title = {Precision in estimating Unruh temperature},

author = {D. Borim and L. C. Céleri and V. I. Kiosses},

url = {http://arxiv.org/abs/2001.09085},

year = {2020},

date = {2020-01-01},

abstract = {The goal of quantum metrology is the exploitation of quantum resources, like entanglement or quantum coherence, in the fundamental task of parameter estimation. Here we consider the question of the estimation of the Unruh temperature in the scenario of relativistic quantum metrology. Specifically, we study two distinct cases. First, a single Unruh-DeWitt detector interacting with a scalar quantum field undergoes an uniform acceleration for a finite amount of proper time, and the role of coherence in the estimation process is analyzed. After this, we consider two initially entangled detectors, one of which is inertial while the other one undergoes acceleration. Our results show that the maximum of the Fisher information, thus characterizing the maximum possible precision according to Crammér-Rao bound, occurs only for small accelerations, while it decreases fast when acceleration increases. Moreover, the role of initial coherence ---in the single detector case---, or entanglement ---in the two detectors case---, is to decrease Fisher information. Therefore, under the considered protocol, internal coherence (or entanglement) is not a resource for estimating Unruh temperature. These unexpected results show that a detection of the Unruh effect can be even more challenge than previously thought. Finally, by considering the connection between Unruh effect and Hawking radiation, we discuss how our results can be understood in the context of the estimation of Hawking temperature.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

### 2019

Pogorzalek, S.; Fedorov, K. G.; Xu, M.; Parra-Rodriguez, A.; Sanz, M.; Fischer, M.; Xie, E.; Inomata, K.; Nakamura, Y.; Solano, E.; Marx, A.; Deppe, F.; Gross, R.

Secure quantum remote state preparation of squeezed microwave states Journal Article

In: Nature Communications, vol. 10, no. 1, pp. 2604, 2019, ISSN: 2041-1723.

@article{Pogorzalek2019a,

title = {Secure quantum remote state preparation of squeezed microwave states},

author = {S. Pogorzalek and K. G. Fedorov and M. Xu and A. Parra-Rodriguez and M. Sanz and M. Fischer and E. Xie and K. Inomata and Y. Nakamura and E. Solano and A. Marx and F. Deppe and R. Gross},

url = {http://www.nature.com/articles/s41467-019-10727-7},

doi = {10.1038/s41467-019-10727-7},

issn = {2041-1723},

year = {2019},

date = {2019-12-01},

journal = {Nature Communications},

volume = {10},

number = {1},

pages = {2604},

publisher = {Nature Publishing Group},

abstract = {Quantum communication protocols based on nonclassical correlations can be more efficient than known classical methods and offer intrinsic security over direct state transfer. In particular, remote state preparation aims at the creation of a desired and known quantum state at a remote location using classical communication and quantum entanglement. We present an experimental realization of deterministic continuous-variable remote state preparation in the microwave regime over a distance of 35 cm. By employing propagating two-mode squeezed microwave states and feedforward, we achieve the remote preparation of squeezed states with up to 1.6 dB of squeezing below the vacuum level. Finally, security of remote state preparation is investigated by using the concept of the one-time pad and measuring the von Neumann entropies. We find nearly identical values for the entropy of the remotely prepared state and the respective conditional entropy given the classically communicated information and, thus, demonstrate close-to-perfect security.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Kokkoniemi, R.; Govenius, J.; Vesterinen, V.; Lake, R. E.; Gunyhó, A. M.; Tan, K. Y.; Simbierowicz, S.; Grönberg, L.; Lehtinen, J.; Prunnila, M.; Hassel, J.; Lamminen, A.; Saira, O. -P.; Möttönen, M.

Nanobolometer with ultralow noise equivalent power Journal Article

In: Communications Physics, vol. 2, no. 1, pp. 124, 2019, ISSN: 2399-3650.

@article{Kokkoniemi2019,

title = {Nanobolometer with ultralow noise equivalent power},

author = {R. Kokkoniemi and J. Govenius and V. Vesterinen and R. E. Lake and A. M. Gunyhó and K. Y. Tan and S. Simbierowicz and L. Grönberg and J. Lehtinen and M. Prunnila and J. Hassel and A. Lamminen and O. -P. Saira and M. Möttönen},

url = {http://www.nature.com/articles/s42005-019-0225-6},

doi = {10.1038/s42005-019-0225-6},

issn = {2399-3650},

year = {2019},

date = {2019-12-01},

journal = {Communications Physics},

volume = {2},

number = {1},

pages = {124},

publisher = {Nature Publishing Group},

abstract = {Since the introduction of bolometers more than a century ago, they have been used in various applications ranging from chemical sensors, consumer electronics, and security to particle physics and astronomy. However, faster bolometers with lower noise are of great interest from the fundamental point of view and to find new use-cases for this versatile concept. We demonstrate a nanobolometer that exhibits roughly an order of magnitude lower noise equivalent power, 20zW/Hz, than previously reported for any bolometer. Importantly, it is more than an order of magnitude faster than other low-noise bolometers, with a time constant of 30 mu s at 60zW/Hz. These results suggest a calorimetric energy resolution of 0.3 zJ = h x 0.4 THz with a time constant of 30 mu s. Further development of this nanobolometer may render it a promising candidate for future applications requiring extremely low noise and high speed such as those in quantum technology and terahertz photon counting.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Chen, Q. -M.; Deppe, F.; Wu, R. -B.; Sun, L.; -x. Liu, Y.; Nojiri, Y.; Pogorzalek, S.; Renger, M.; Partanen, M.; Fedorov, K. G.; Marx, A.; Gross, R.

Quantum Fourier Transform in Oscillating Modes Miscellaneous

2019.

@misc{Chen2019a,

title = {Quantum Fourier Transform in Oscillating Modes},

author = {Q. -M. Chen and F. Deppe and R. -B. Wu and L. Sun and Y. -x. Liu and Y. Nojiri and S. Pogorzalek and M. Renger and M. Partanen and K. G. Fedorov and A. Marx and R. Gross},

url = {http://arxiv.org/abs/1912.09861},

year = {2019},

date = {2019-12-01},

abstract = {Quantum Fourier transform (QFT) is a key ingredient of many quantum algorithms. In typical applications such as phase estimation, a considerable number of ancilla qubits and gates are used to form a Hilbert space large enough for high-precision results. Qubit recycling reduces the number of ancilla qubits to one, but it is only applicable to semi-classical QFT and requires repeated measurements and feedforward within the coherence time of the qubits. In this work, we explore a novel approach based on resonators that forms a high-dimensional Hilbert space for the realization of QFT. By employing the perfect state-transfer method, we map an unknown multi-qubit state to a single resonator, and obtain the QFT state in the second oscillator through cross-Kerr interaction and projective measurement. A quantitive analysis shows that our method allows for high-dimensional and fully-quantum QFT employing the state-of-the-art superconducting quantum circuits. This paves the way for implementing various QFT related quantum algorithms.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Barrios, G. A.; Retamal, J. C.; Solano, E.; Sanz, M.

Analog simulator of integro-differential equations with classical memristors Journal Article

In: Scientific Reports, vol. 9, no. 1, pp. 12928, 2019, ISSN: 2045-2322.

@article{Barrios2019,

title = {Analog simulator of integro-differential equations with classical memristors},

author = {G. A. Barrios and J. C. Retamal and E. Solano and M. Sanz},

doi = {10.1038/s41598-019-49204-y},

issn = {2045-2322},

year = {2019},

date = {2019-12-01},

journal = {Scientific Reports},

volume = {9},

number = {1},

pages = {12928},

publisher = {Nature Publishing Group},

abstract = {An analog computer makes use of continuously changeable quantities of a system, such as its electrical, mechanical, or hydraulic properties, to solve a given problem. While these devices are usually computationally more powerful than their digital counterparts, they suffer from analog noise which does not allow for error control. We will focus on analog computers based on active electrical networks comprised of resistors, capacitors, and operational amplifiers which are capable of simulating any linear ordinary differential equation. However, the class of nonlinear dynamics they can solve is limited. In this work, by adding memristors to the electrical network, we show that the analog computer can simulate a large variety of linear and nonlinear integro-differential equations by carefully choosing the conductance and the dynamics of the memristor state variable. We study the performance of these analog computers by simulating integro-differential models related to fluid dynamics, nonlinear Volterra equations for population growth, and quantum models describing non-Markovian memory effects, among others. Finally, we perform stability tests by considering imperfect analog components, obtaining robust solutions with up to 13% relative error for relevant timescales.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Eneriz, H.; Rossatto, D. Z.; Cárdenas-López, F. A.; Solano, E.; Sanz, M.

Degree of Quantumness in Quantum Synchronization Journal Article

In: Scientific Reports, vol. 9, no. 1, pp. 19933, 2019, ISSN: 2045-2322.

@article{Eneriz2019,

title = {Degree of Quantumness in Quantum Synchronization},

author = {H. Eneriz and D. Z. Rossatto and F. A. Cárdenas-López and E. Solano and M. Sanz},

url = {http://www.nature.com/articles/s41598-019-56468-x},

doi = {10.1038/s41598-019-56468-x},

issn = {2045-2322},

year = {2019},

date = {2019-12-01},

journal = {Scientific Reports},

volume = {9},

number = {1},

pages = {19933},

publisher = {Nature Research},

abstract = {We introduce the concept of degree of quantumness in quantum synchronization, a measure of the quantum nature of synchronization in quantum systems. Following techniques from quantum information, we propose the number of non-commuting observables that synchronize as a measure of quantumness. This figure of merit is compatible with already existing synchronization measurements, and it captures different physical properties. We illustrate it in a quantum system consisting of two weakly interacting cavity-qubit systems, which are coupled via the exchange of bosonic excitations between the cavities. Moreover, we study the synchronization of the expectation values of the Pauli operators and we propose a feasible superconducting circuit setup. Finally, we discuss the degree of quantumness in the synchronization between two quantum van der Pol oscillators.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Gutiérrez, M. J.; Berrocal, J.; Domínguez, F.; Arrazola, I.; Block, M.; Solano, E.; Rodríguez, D.

Dynamics of an unbalanced two-ion crystal in a Penning trap for application in optical mass spectrometry Journal Article

In: Physical Review A, vol. 100, no. 6, pp. 063415, 2019, ISSN: 2469-9926.

@article{Gutierrez2019,

title = {Dynamics of an unbalanced two-ion crystal in a Penning trap for application in optical mass spectrometry},

author = {M. J. Gutiérrez and J. Berrocal and F. Domínguez and I. Arrazola and M. Block and E. Solano and D. Rodríguez},

url = {https://link.aps.org/doi/10.1103/PhysRevA.100.063415},

doi = {10.1103/PhysRevA.100.063415},

issn = {2469-9926},

year = {2019},

date = {2019-12-01},

journal = {Physical Review A},

volume = {100},

number = {6},

pages = {063415},

publisher = {American Physical Society},

abstract = {In this paper, the dynamics of an unbalanced two-ion crystal comprising the "target" and the "sensor" ions confined in a Penning trap along the magnetic-field axis has been studied. First, the low amplitude regime is addressed. In this regime, the overall potential including the Coulomb repulsion between the ions can be considered harmonic and the axial, magnetron, and reduced-cyclotron modes split up into the so-called stretch and common modes, that are generalizations of the well-known "breathing" and "center-of-mass" motions of a balanced crystal made of two ions. By using optical detection to measure the frequencies of the modes of the crystal, and of the sensor ion on its own, in the quantum regime of motion, it will be possible to determine the target ion's free-cyclotron frequency. The nonharmonicity of the Coulomb interaction is also discussed since this causes large systematic effects, which are minimized due to the high sensitivity of the optical detection method when the crystal is cooled to the ground state of motion in the Penning trap.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Peng, J.; Rico, E.; Zhong, J.; Solano, E.; Egusquiza, I. L.

Unified superradiant phase transitions Journal Article

In: Physical Review A, vol. 100, no. 6, pp. 063820, 2019, ISSN: 2469-9926.

@article{Peng2019,

title = {Unified superradiant phase transitions},

author = {J. Peng and E. Rico and J. Zhong and E. Solano and I. L. Egusquiza},

url = {https://link.aps.org/doi/10.1103/PhysRevA.100.063820},

doi = {10.1103/PhysRevA.100.063820},

issn = {2469-9926},

year = {2019},

date = {2019-12-01},

journal = {Physical Review A},

volume = {100},

number = {6},

pages = {063820},

publisher = {American Physical Society},

abstract = {We prove that superradiant phase transitions (SPTs) of the Dicke model and its generalizations in the thermodynamic and classical oscillator limit are indeed of the same type. In this sense, we unify SPTs under both limits at zero and finite temperature. We show that the mean-field approximation for bosons is exact in both cases, and compute the structure and location of the phase transitions in parameter space using a concise analytic method. Moreover, we illustrate how SPT properties (first order, second order, or none) are related to symmetry. Finally, we uncover general features of the phase structure in the space of parameters of these models with dipolar couplings. There will be a region of normal phase in the neighborhood of the origin of the space of dipolar couplings, and that generally one flows radially in this space to a superradiant phase.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Mäkinen, A.; Ikonen, J.; Partanen, M.; Möttönen, M.

Reconstruction approach to quantum dynamics of bosonic systems Journal Article

In: Physical Review A, vol. 100, no. 4, pp. 042109, 2019, ISSN: 2469-9926.

@article{Maekinen2019,

title = {Reconstruction approach to quantum dynamics of bosonic systems},

author = {A. Mäkinen and J. Ikonen and M. Partanen and M. Möttönen},

url = {https://link.aps.org/doi/10.1103/PhysRevA.100.042109},

doi = {10.1103/PhysRevA.100.042109},

issn = {2469-9926},

year = {2019},

date = {2019-10-01},

journal = {Physical Review A},

volume = {100},

number = {4},

pages = {042109},

publisher = {American Physical Society},

abstract = {We propose an approach to analytically solve the quantum dynamics of bosonic systems. The method is based on reconstructing the quantum state of the system from the moments of its annihilation operators, dynamics of which is solved in the Heisenberg picture. The proposed dynamical reconstruction method is general in the sense that it does not require assumptions on the initial conditions of the system such as separability, or the structure of the system such as linearity. It is an alternative to the standard master-equation approaches, which are analytically demanding especially for large multipartite quantum systems. To demonstrate the proposed technique, we apply it to a system consisting of two coupled damped quantum harmonic oscillators.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Partanen, M.; Goetz, J.; Tan, K. Y.; Kohvakka, K.; Sevriuk, V.; Lake, R. E.; Kokkoniemi, R.; Ikonen, J.; Hazra, D.; Mäkinen, A.; Hyyppä, E.; Grönberg, L.; Vesterinen, V.; Silveri, M.; Möttönen, M.

Exceptional points in tunable superconducting resonators Journal Article

In: Physical Review B, vol. 100, no. 13, pp. 134505, 2019, ISSN: 2469-9950.

@article{Partanen2019,

title = {Exceptional points in tunable superconducting resonators},

author = {M. Partanen and J. Goetz and K. Y. Tan and K. Kohvakka and V. Sevriuk and R. E. Lake and R. Kokkoniemi and J. Ikonen and D. Hazra and A. Mäkinen and E. Hyyppä and L. Grönberg and V. Vesterinen and M. Silveri and M. Möttönen},

url = {https://link.aps.org/doi/10.1103/PhysRevB.100.134505},

doi = {10.1103/PhysRevB.100.134505},

issn = {2469-9950},

year = {2019},

date = {2019-10-01},

journal = {Physical Review B},

volume = {100},

number = {13},

pages = {134505},

publisher = {American Physical Society},

abstract = {Superconducting quantum circuits are potential candidates to realize a large-scale quantum computer. The envisioned large density of integrated components, however, requires a proper thermal management and control of dissipation. To this end, it is advantageous to utilize tunable dissipation channels and to exploit the optimized heat flow at exceptional points (EPs). Here, we experimentally realize an EP in a superconducting microwave circuit consisting of two resonators. The EP is a singularity point of the effective Hamiltonian, and corresponds to critical damping with the most efficient heat transfer between the resonators without back and forth oscillation of energy. We observe a crossover from underdamped to overdamped coupling across the EP by utilizing photon-assisted tunneling as an in situ tunable dissipative element in one of the resonators. These methods can be used to obtain fast dissipation, for example, for initializing qubits to their ground states. In addition, these results pave the way for thorough investigation of parity-time symmetry and the spontaneous symmetry breaking at the EP in superconducting quantum circuits operating at the level of single energy quanta.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Sevriuk, V. A.; Tan, K. Y.; Hyyppä, E.; Silveri, M.; Partanen, M.; Jenei, M.; Masuda, S.; Goetz, J.; Vesterinen, V.; Grönberg, L.; Möttönen, M.

Fast control of dissipation in a superconducting resonator Journal Article

In: Applied Physics Letters, vol. 115, no. 8, pp. 082601, 2019, ISSN: 0003-6951.

@article{Sevriuk2019,

title = {Fast control of dissipation in a superconducting resonator},

author = {V. A. Sevriuk and K. Y. Tan and E. Hyyppä and M. Silveri and M. Partanen and M. Jenei and S. Masuda and J. Goetz and V. Vesterinen and L. Grönberg and M. Möttönen},

url = {http://aip.scitation.org/doi/10.1063/1.5116659},

doi = {10.1063/1.5116659},

issn = {0003-6951},

year = {2019},

date = {2019-08-01},

journal = {Applied Physics Letters},

volume = {115},

number = {8},

pages = {082601},

publisher = {American Institute of Physics Inc.},

abstract = {We report on fast tunability of an electromagnetic environment coupled to a superconducting coplanar waveguide resonator. Namely, we utilize a recently developed quantum-circuit refrigerator (QCR) to experimentally demonstrate a dynamic tunability in the total damping rate of the resonator up to almost two orders of magnitude. Based on the theory, it corresponds to a change in the internal damping rate by nearly four orders of magnitude. The control of the QCR is fully electrical, with the shortest implemented operation times in the range of 10 ns. This experiment constitutes a fast active reset of a superconducting quantum circuit. In the future, a similar scheme can potentially be used to initialize superconducting quantum bits.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Gonzalez-Raya, T.; Cheng, X. H.; Egusquiza, I. L.; Chen, X.; Sanz, M.; Solano, E.

Quantized Single-Ion-Channel Hodgkin-Huxley Model for Quantum Neurons Journal Article

In: Physical Review Applied, vol. 12, no. 1, pp. 014037, 2019, ISSN: 2331-7019.

@article{GonzalezRaya2019,

title = {Quantized Single-Ion-Channel Hodgkin-Huxley Model for Quantum Neurons},

author = {T. Gonzalez-Raya and X. H. Cheng and I. L. Egusquiza and X. Chen and M. Sanz and E. Solano},

doi = {10.1103/PhysRevApplied.12.014037},

issn = {2331-7019},

year = {2019},

date = {2019-07-01},

journal = {Physical Review Applied},

volume = {12},

number = {1},

pages = {014037},

publisher = {American Physical Society},

abstract = {The Hodgkin-Huxley model describes the behavior of the cell membrane in neurons, treating each part of it as an electric circuit element; namely, capacitors, memristors, and voltage sources. We focus on the activation channel of potassium ions, due to its simplicity, while keeping most of the features displayed by the original model. This reduced version is essentially a classical memristor, a resistor whose resistance depends on the history of electric signals that have crossed it, coupled to a voltage source and a capacitor. We consider a quantized Hodgkin-Huxley model based on a quantum-memristor formalism. We compare the behavior of the membrane voltage and the potassium-channel conductance when the circuit is subjected to ac sources, in both the classical realm and the quantum realm. Numerical simulations show an expected adaptation of the considered channel conductance depending on the signal history in all regimes. Remarkably, the computation of higher moments of the voltage shows purely quantum features related to the circuit zero-point energy. Finally, we study the implementation of the Hodgkin-Huxley quantum memristor as an asymmetric rf superconducting quantum-interference device in superconducting circuits. This study may allow the construction of quantum neuron networks inspired by the brain function, as well as the design of neuromorphic quantum architectures for quantum machine learning.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Mera, B.; Sacha, K.; Omar, Y.

Topologically Protected Quantization of Work Journal Article

In: Physical Review Letters, vol. 123, no. 2, pp. 020601, 2019, ISSN: 0031-9007.

@article{Mera2019,

title = {Topologically Protected Quantization of Work},

author = {B. Mera and K. Sacha and Y. Omar},

url = {https://link.aps.org/doi/10.1103/PhysRevLett.123.020601},

doi = {10.1103/PhysRevLett.123.020601},

issn = {0031-9007},

year = {2019},

date = {2019-07-01},

journal = {Physical Review Letters},

volume = {123},

number = {2},

pages = {020601},

publisher = {American Physical Society},

abstract = {The transport of a particle in the presence of a potential that changes periodically in space and in time can be characterized by the amount of work needed to shift a particle by a single spatial period of the potential. In general, this amount of work, when averaged over a single temporal period of the potential, can take any value in a continuous fashion. Here, we present a topological effect inducing the quantization of the average work. We find that this work is equal to the first Chern number calculated in a unit cell of a space-time lattice. Hence, this quantization of the average work is topologically protected. We illustrate this phenomenon with the example of an atom whose center of mass motion is coupled to its internal degrees of freedom by electromagnetic waves.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Arrazola, I.; Solano, E.; Casanova, J.

Selective hybrid spin interactions with low radiation power Journal Article

In: Physical Review B, vol. 99, no. 24, pp. 245405, 2019, ISSN: 2469-9950.

@article{Arrazola2019,

title = {Selective hybrid spin interactions with low radiation power},

author = {I. Arrazola and E. Solano and J. Casanova},

url = {https://link.aps.org/doi/10.1103/PhysRevB.99.245405},

doi = {10.1103/PhysRevB.99.245405},

issn = {2469-9950},

year = {2019},

date = {2019-06-01},

journal = {Physical Review B},

volume = {99},

number = {24},

pages = {245405},

publisher = {American Physical Society},

abstract = {We present a protocol for designing appropriately extended $pi$ pulses that achieves tunable, thus selective, electron-nuclear spin interactions with low-driving radiation power. The latter is of great benefit when $pi$ pulses are displayed over biological samples as it reduces sample heating. Our method is general since it can be applied to different quantum sensor devices such as nitrogen vacancy centers or silicon vacancy centers. Furthermore, it can be directly incorporated in commonly used stroboscopic dynamical decoupling techniques to achieve enhanced nuclear selectivity and control, which demonstrates its flexibility.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Albarrán-Arriagada, F.; Retamal, J. C.; Solano, E.; Lamata, L.

Reinforcement learning for semi-autonomous approximate quantum eigensolver Journal Article

In: Machine Learning: Science and Technology, vol. 1, no. 1, pp. 015002, 2019.

@article{AlbarranArriagada2019,

title = {Reinforcement learning for semi-autonomous approximate quantum eigensolver},

author = {F. Albarrán-Arriagada and J. C. Retamal and E. Solano and L. Lamata},

doi = {10.1088/2632-2153/ab43b4},

year = {2019},

date = {2019-06-01},

journal = {Machine Learning: Science and Technology},

volume = {1},

number = {1},

pages = {015002},

publisher = {IOP Publishing},

abstract = {The characterization of an operator by its eigenvectors and eigenvalues allows us to know its action over any quantum state. Here, we propose a protocol to obtain an approximation of the eigenvectors of an arbitrary Hermitian quantum operator. This protocol is based on measurement and feedback processes, which characterize a reinforcement learning protocol. Our proposal is composed of two systems, a black box named environment and a quantum state named agent. The role of the environment is to change any quantum state by a unitary matrix $hatU_E=e^-itauhatmathcalO_E$ where $hatmathcalO_E$ is a Hermitian operator, and $tau$ is a real parameter. The agent is a quantum state which adapts to some eigenvector of $hatmathcalO_E$ by repeated interactions with the environment, feedback process, and semi-random rotations. With this proposal, we can obtain an approximation of the eigenvectors of a random qubit operator with average fidelity over 90% in less than 10 iterations, and surpass 98% in less than 300 iterations. Moreover, for the two-qubit cases, the four eigenvectors are obtained with fidelities above 89% in 8000 iterations for a random operator, and fidelities of $99%$ for an operator with the Bell states as eigenvectors. This protocol can be useful to implement semi-autonomous quantum devices which should be capable of extracting information and deciding with minimal resources and without human intervention.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Alaeian, H.; Chang, C. W. S.; Moghaddam, M. V.; Wilson, C. M.; Solano, E.; Rico, E.

Creating lattice gauge potentials in circuit QED: The bosonic Creutz ladder Journal Article

In: Physical Review A, vol. 99, no. 5, pp. 053834, 2019, ISSN: 2469-9926.

@article{Alaeian2019,

title = {Creating lattice gauge potentials in circuit QED: The bosonic Creutz ladder},

author = {H. Alaeian and C. W. S. Chang and M. V. Moghaddam and C. M. Wilson and E. Solano and E. Rico},

url = {https://link.aps.org/doi/10.1103/PhysRevA.99.053834},

doi = {10.1103/PhysRevA.99.053834},

issn = {2469-9926},

year = {2019},

date = {2019-05-01},

journal = {Physical Review A},

volume = {99},

number = {5},

pages = {053834},

publisher = {American Physical Society},

abstract = {In this work we propose two protocols to make an effective gauge potential for microwave photons in circuit QED. The first scheme is based on coupled transmons whose on-site energies are harmonically modulated in time. We investigate the effect of various types of capacitive and inductive couplings, and the role of the phase difference between adjacent sites on creating a complex hopping rate between coupled qubits. The second method relies on the parametrically coupling the modes of a SQUID in a resonator and controlling the hopping phase via a coherent pump. Both proposals can be readily realized in a superconducting circuit with the existing technology and are suitable for scalable lattices. As an example benefiting from these complex-valued hopping terms, we simulated the behavior of a plaquette of bosonic Creutz ladder as one of the important models with interdisciplinary interest in various branches of physics. Our results clearly show the emergence of chiral edge modes and directional transport between lattice sites. Combined with intrinsic nonlinearity of the transmon qubits such lattices would be an ideal platform for simulating many different Hamiltonians such as the Bose-Hubbard model with nontrivial gauge fields. Important direct applications of the presented results span a broad range from signal processing in nonreciprocal transport to quantum simulation of gauge-invariant models in fundamental physics.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Agustí, A.; Solano, E.; Sabín, C.

Entanglement through qubit motion and the dynamical Casimir effect Journal Article

In: Physical Review A, vol. 99, no. 5, pp. 052328, 2019, ISSN: 2469-9926.

@article{Agusti2019,

title = {Entanglement through qubit motion and the dynamical Casimir effect},

author = {A. Agustí and E. Solano and C. Sabín},

doi = {10.1103/PhysRevA.99.052328},

issn = {2469-9926},

year = {2019},

date = {2019-05-01},

journal = {Physical Review A},

volume = {99},

number = {5},

pages = {052328},

publisher = {American Physical Society},

abstract = {We explore the interplay between acceleration radiation and the dynamical Casimir effect in the field of superconducting quantum technologies, analyzing the generation of entanglement between two qubits by means of the dynamical Casimir effect in several states of qubit motion. We show that the correlated absorption and emission of photons are crucial for entanglement, which in some cases can be linked to the notion of simultaneity in special relativity.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Puebla, R.; Zicari, G.; Arrazola, I.; Solano, E.; Paternostro, M.; Casanova, J.

Spin-Boson Model as A Simulator of Non-Markovian Multiphoton Jaynes-Cummings Models Journal Article

In: Symmetry, vol. 11, no. 5, pp. 695, 2019, ISSN: 2073-8994.

@article{Puebla2019,

title = {Spin-Boson Model as A Simulator of Non-Markovian Multiphoton Jaynes-Cummings Models},

author = {R. Puebla and G. Zicari and I. Arrazola and E. Solano and M. Paternostro and J. Casanova},

url = {https://www.mdpi.com/2073-8994/11/5/695},

doi = {10.3390/sym11050695},

issn = {2073-8994},

year = {2019},

date = {2019-05-01},

journal = {Symmetry},

volume = {11},

number = {5},

pages = {695},

publisher = {MDPI AG},

abstract = {The paradigmatic spin-boson model considers a spin degree of freedom interacting with an environment typically constituted by a continuum of bosonic modes. This ubiquitous model is of relevance in a number of physical systems where, in general, one has neither control over the bosonic modes, nor the ability to tune distinct interaction mechanisms. Despite this apparent lack of control, we present a suitable transformation that approximately maps the spin-boson dynamics into that of a tunable multiphoton Jaynes-Cummings model undergoing dissipation. Interestingly, the latter model describes the coherent interaction between a spin and a single bosonic mode via the simultaneous exchange of n bosons per spin excitation. Resorting to the so-called reaction coordinate method, we identify a relevant collective bosonic mode in the environment, which is then used to generate multiphoton interactions following the proposed theoretical framework. Moreover, we show that spin-boson models featuring structured environments can lead to non-Markovian multiphoton Jaynes-Cummings dynamics. We discuss the validity of the proposed method depending on the parameters and analyse its performance, which is supported by numerical simulations. In this manner, the spin-boson model serves as a good analogue quantum simulator for the inspection and realization of multiphoton Jaynes-Cummings models, as well as the interplay of non-Markovian effects and, thus, as a simulator of light-matter systems with tunable interaction mechanisms.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Ding, Y.; Lamata, L.; Martín-Guerrero, J. D.; Lizaso, E.; Mugel, S.; Chen, Xi; Orús, R.; Solano, E.; Sanz, M.

Towards Prediction of Financial Crashes with a D-Wave Quantum Computer Miscellaneous

2019.

@misc{Ding2019a,

title = {Towards Prediction of Financial Crashes with a D-Wave Quantum Computer},

author = {Y. Ding and L. Lamata and J. D. Martín-Guerrero and E. Lizaso and S. Mugel and Xi Chen and R. Orús and E. Solano and M. Sanz},

url = {http://arxiv.org/abs/1904.05808},

year = {2019},

date = {2019-04-01},

abstract = {Prediction of financial crashes in a complex financial network is known to be an NP-hard problem, i.e., a problem which cannot be solved efficiently with a classical computer. We experimentally explore a novel approach to this problem by using a D-Wave quantum computer to obtain financial equilibrium more efficiently. To be specific, the equilibrium condition of a nonlinear financial model is embedded into a higher-order unconstrained binary optimization (HUBO) problem, which is then transformed to a spin-$1/2$ Hamiltonian with at most two-qubit interactions. The problem is thus equivalent to finding the ground state of an interacting spin Hamiltonian, which can be approximated with a quantum annealer. Our experiment paves the way to study quantitative macroeconomics, enlarging the number of problems that can be handled by current quantum computers.},

keywords = {},

pubstate = {published},

tppubtype = {misc}

}

Puebla, R.; Casanova, J.; Houhou, O.; Solano, E.; Paternostro, M.

Quantum simulation of multiphoton and nonlinear dissipative spin-boson models Journal Article

In: Physical Review A, vol. 99, no. 3, pp. 032303, 2019, ISSN: 2469-9926.

@article{Puebla2019a,

title = {Quantum simulation of multiphoton and nonlinear dissipative spin-boson models},

author = {R. Puebla and J. Casanova and O. Houhou and E. Solano and M. Paternostro},

url = {https://link.aps.org/doi/10.1103/PhysRevA.99.032303},

doi = {10.1103/PhysRevA.99.032303},

issn = {2469-9926},

year = {2019},

date = {2019-03-01},

journal = {Physical Review A},

volume = {99},

number = {3},

pages = {032303},

publisher = {American Physical Society},

abstract = {We present a framework for the realization of dissipative evolutions of spin-boson models, including multiphoton exchange dynamics, as well as nonlinear transition rates. Our approach is based on the implementation of a generalized version of a dissipative linear quantum Rabi model. The latter comprises a linearly coupled spin-boson term, spin rotations, and standard dissipators. We provide numerical simulations of illustrative cases supporting the good performance of our method. Our work allows for the simulation of a large class of fundamentally different quantum models where the effect of distinct dissipative processes can be easily investigated.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Cárdenas-López, F.; Romero, G.; Lamata, L.; Solano, E.; Retamal, J. C.

Parity-Assisted Generation of Nonclassical States of Light in Circuit Quantum Electrodynamics Journal Article

In: Symmetry, vol. 11, no. 3, pp. 372, 2019, ISSN: 2073-8994.

@article{CardenasLopez2019,

title = {Parity-Assisted Generation of Nonclassical States of Light in Circuit Quantum Electrodynamics},

author = {F. Cárdenas-López and G. Romero and L. Lamata and E. Solano and J. C. Retamal},

url = {https://www.mdpi.com/2073-8994/11/3/372},

doi = {10.3390/sym11030372},

issn = {2073-8994},

year = {2019},

date = {2019-03-01},

journal = {Symmetry},

volume = {11},

number = {3},

pages = {372},

publisher = {Multidisciplinary Digital Publishing Institute},

abstract = {We propose a method to generate nonclassical states of light in multimode microwave cavities. Our approach considers two-photon processes that take place in a system composed of N extended cavities and an ultrastrongly coupled light-matter system. Under specific resonance conditions, our method generates, in a deterministic manner, product states of uncorrelated photon pairs, Bell states, and W states in different modes on the extended cavities. Furthermore, the numerical simulations show that the generation scheme exhibits a collective effect which decreases the generation time in the same proportion as the number of extended cavity increases. Moreover, the entanglement encoded in the photonic states can be transferred towards ancillary two-level systems to generate genuine multipartite entanglement. Finally, we discuss the feasibility of our proposal in circuit quantum electrodynamics. This proposal could be of interest in the context of quantum random number generator, due to the quadratic scaling of the output state.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Yu, S.; Albarrán-Arriagada, F.; Retamal, J. C.; Wang, Y. -T.; Liu, W.; Ke, Z. -J.; Meng, Y.; Li, Z. -P.; Tang, J. -S.; Solano, E.; Lamata, L.; Li, C. -F.; Guo, G. -C.

Reconstruction of a Photonic Qubit State with Reinforcement Learning Journal Article

In: Advanced Quantum Technologies, pp. 1800074, 2019, ISSN: 2511-9044.

@article{Yu2019,

title = {Reconstruction of a Photonic Qubit State with Reinforcement Learning},

author = {S. Yu and F. Albarrán-Arriagada and J. C. Retamal and Y. -T. Wang and W. Liu and Z. -J. Ke and Y. Meng and Z. -P. Li and J. -S. Tang and E. Solano and L. Lamata and C. -F. Li and G. -C. Guo},

url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/qute.201800074},

doi = {10.1002/qute.201800074},

issn = {2511-9044},

year = {2019},

date = {2019-03-01},

journal = {Advanced Quantum Technologies},

pages = {1800074},

publisher = {John Wiley & Sons, Ltd},

abstract = {An experiment is performed to reconstruct an unknown photonic quantum state with a limited amount of copies. A semi-quantum reinforcement learning approach is employed to adapt one qubit state, an "agent," to an unknown quantum state, an "environment," by successive single-shot measurements and feedback, in order to achieve maximum overlap. The experimental learning device herein, composed of a quantum photonics setup, can adjust the corresponding parameters to rotate the agent system based on the measurement outcomes "0" or "1" in the environment (i.e., reward/punishment signals). The results show that, when assisted by such a quantum machine learning technique, fidelities of the deterministic single-photon agent states can achieve over 88% under a proper reward/punishment ratio within 50 iterations. This protocol offers a tool for reconstructing an unknown quantum state when only limited copies are provided, and can also be extended to higher dimensions, multipartite, and mixed quantum state scenarios.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Ding, Y.; Lamata, L.; Sanz, M.; Chen, X.; Solano, E.

Experimental Implementation of a Quantum Autoencoder via Quantum Adders Journal Article

In: Advanced Quantum Technologies, pp. 1800065, 2019, ISSN: 2511-9044.

@article{Ding2019,

title = {Experimental Implementation of a Quantum Autoencoder via Quantum Adders},

author = {Y. Ding and L. Lamata and M. Sanz and X. Chen and E. Solano},

url = {http://doi.wiley.com/10.1002/qute.201800065},

doi = {10.1002/qute.201800065},

issn = {2511-9044},

year = {2019},

date = {2019-02-01},

journal = {Advanced Quantum Technologies},

pages = {1800065},

publisher = {John Wiley & Sons, Ltd},

abstract = {Quantum autoencoders allow for reducing the amount of resources in a quantum computation by mapping the original Hilbert space onto a reduced space with the relevant information. Recently, it was proposed to employ approximate quantum adders to implement quantum autoencoders in quantum technologies. Here, we carry out the experimental implementation of this proposal in the Rigetti cloud quantum computer employing up to three qubits. The experimental fidelities are in good agreement with the theoretical prediction, thus proving the feasibility to realize quantum autoencoders via quantum adders in state-of-the-art superconducting quantum technologies.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Parra-Rodriguez, A.; Egusquiza, I. L.; DiVincenzo, D. P.; Solano, E.

Canonical circuit quantization with linear nonreciprocal devices Journal Article

In: Physical Review B, vol. 99, no. 1, pp. 014514, 2019.

@article{ParraRodriguez2019,

title = {Canonical circuit quantization with linear nonreciprocal devices},

author = {A. Parra-Rodriguez and I. L. Egusquiza and D. P. DiVincenzo and E. Solano},

url = {https://journals.aps.org/prb/abstract/10.1103/PhysRevB.99.014514},

doi = {10.1103/PhysRevB.99.014514},

year = {2019},

date = {2019-01-01},

journal = {Physical Review B},

volume = {99},

number = {1},

pages = {014514},

publisher = {American Physical Society},

abstract = {Nonreciprocal devices effectively mimic the breaking of time-reversal symmetry for the subspace of dynamical variables that they couple, and can be used to create chiral information processing networks. We study the systematic inclusion of ideal gyrators and circulators into Lagrangian and Hamiltonian descriptions of lumped-element electrical networks. The proposed theory is of wide applicability in general nonreciprocal networks on the quantum regime. We apply it to pedagogical and pathological examples of circuits containing Josephson junctions and ideal nonreciprocal elements described by admittance matrices, and compare it with the more involved treatment of circuits based on nonreciprocal devices characterized by impedance or scattering matrices. Finally, we discuss the dual quantization of circuits containing phase-slip junctions and nonreciprocal devices.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Casanova, J.; Torrontegui, E.; Plenio, M. B.; García-Ripoll, J. J.; Solano, E.

Modulated Continuous Wave Control for Energy-Efficient Electron-Nuclear Spin Coupling Journal Article

In: Physical Review Letters, vol. 122, no. 1, pp. 010407, 2019, ISSN: 0031-9007.

@article{Casanova2019,

title = {Modulated Continuous Wave Control for Energy-Efficient Electron-Nuclear Spin Coupling},

author = {J. Casanova and E. Torrontegui and M. B. Plenio and J. J. García-Ripoll and E. Solano},

url = {https://link.aps.org/doi/10.1103/PhysRevLett.122.010407},

doi = {10.1103/PhysRevLett.122.010407},

issn = {0031-9007},

year = {2019},

date = {2019-01-01},

journal = {Physical Review Letters},

volume = {122},

number = {1},

pages = {010407},

publisher = {American Physical Society},

abstract = {We develop energy efficient, continuous microwave schemes to couple electron and nuclear spins, using phase or amplitude modulation to bridge their frequency difference. These controls have promising applications in biological systems, where microwave power should be limited, as well as in situations with high Larmor frequencies due to large magnetic fields and nuclear magnetic moments. These include nanoscale NMR where high magnetic fields achieves enhanced thermal nuclear polarization and larger chemical shifts. Our controls are also suitable for quantum information processors and nuclear polarization schemes.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Cárdenas-López, F. A.; Sanz, M.; Retamal, J. C.; Solano, E.

Enhanced Quantum Synchronization via Quantum Machine Learning Journal Article

In: Advanced Quantum Technologies, pp. 1800076, 2019, ISSN: 2511-9044.

@article{CardenasLopez2019a,

title = {Enhanced Quantum Synchronization via Quantum Machine Learning},

author = {F. A. Cárdenas-López and M. Sanz and J. C. Retamal and E. Solano},

url = {http://doi.wiley.com/10.1002/qute.201800076},

doi = {10.1002/qute.201800076},

issn = {2511-9044},

year = {2019},

date = {2019-01-01},

journal = {Advanced Quantum Technologies},

pages = {1800076},

publisher = {John Wiley & Sons, Ltd},

abstract = {We study the quantum synchronization between a pair of two-level systems inside two coupled cavities. By using a digital-analog decomposition of the master equation that rules the system dynamics, we show that this approach leads to quantum synchronization between both two-level systems. Moreover, we can identify in this digital-analog block decomposition the fundamental elements of a quantum machine learning protocol, in which the agent and the environment (learning units) interact through a mediating system, namely, the register. If we can additionally equip this algorithm with a classical feedback mechanism, which consists of projective measurements in the register, reinitialization of the register state and local conditional operations on the agent and environment subspace, a powerful and flexible quantum machine learning protocol emerges. Indeed, numerical simulations show that this protocol enhances the synchronization process, even when every subsystem experience different loss/decoherence mechanisms, and give us the flexibility to choose the synchronization state. Finally, we propose an implementation based on current technologies in superconducting circuits.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

### 2018

Capela, M.; Sanz, M.; Solano, E.; Céleri, L. C.

Kolmogorov-Sinai entropy and dissipation in driven classical Hamiltonian systems Journal Article

In: Physical Review E, vol. 98, no. 5, pp. 052109, 2018, ISSN: 2470-0053.

@article{Capela2018,

title = {Kolmogorov-Sinai entropy and dissipation in driven classical Hamiltonian systems},

author = {M. Capela and M. Sanz and E. Solano and L. C. Céleri},

doi = {10.1103/PhysRevE.98.052109},

issn = {2470-0053},

year = {2018},

date = {2018-11-01},

journal = {Physical Review E},

volume = {98},

number = {5},

pages = {052109},

publisher = {American Physical Society},

abstract = {A central concept in the connection between physics and information theory is entropy, which represents the amount of information extracted from the system by the observer performing measurements in an experiment. Indeed, Jaynes' principle of maximum entropy allows to establish the connection between entropy in statistical mechanics and information entropy. In this sense, the dissipated energy in a classical Hamiltonian process, known as the thermodynamic entropy production, is connected to the relative entropy between the forward and backward probability densities. Recently, it was revealed that energetic inefficiency and model inefficiency, defined as the difference in mutual information that the system state shares with the future and past environmental variables, are equivalent concepts in Markovian processes. As a consequence, the question about a possible connection between model unpredictability and energetic inefficiency in the framework of classical physics emerges. Here, we address this question by connecting the concepts of random behavior of a classical Hamiltonian system, the Kolmogorov-Sinai entropy, with its energetic inefficiency, the dissipated work. This approach allows us to provide meaningful interpretations of information concepts in terms of thermodynamic quantities.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

Sanz, M.; Fedorov, K. G.; Deppe, F.; Solano, E.

Challenges in Open-air Microwave Quantum Communication and Sensing Inproceedings

In: 2018 IEEE Conference on Antenna Measurements & Applications (CAMA), pp. 1–4, IEEE, 2018, ISBN: 978-1-5386-5795-9.

@inproceedings{Sanz2018,

title = {Challenges in Open-air Microwave Quantum Communication and Sensing},

author = {M. Sanz and K. G. Fedorov and F. Deppe and E. Solano},

url = {https://ieeexplore.ieee.org/document/8530599/},

doi = {10.1109/CAMA.2018.8530599},

isbn = {978-1-5386-5795-9},

year = {2018},

date = {2018-09-01},

booktitle = {2018 IEEE Conference on Antenna Measurements & Applications (CAMA)},

pages = {1--4},

publisher = {IEEE},

abstract = {Quantum communication is a holy grail to achieve secure communication among a set of partners, since it is provably unbreakable by physical laws. Quantum sensing employs quantum entanglement as an extra resource to determine parameters by either using less resources or attaining a precision unachievable in classical protocols. A paradigmatic example is the quantum radar, which allows one to detect an object without being detected oneself, by making use of the additional asset provided by quantum entanglement to reduce the intensity of the signal. In the optical regime, impressive technological advances have been reached in the last years, such as the first quantum communication between ground and satellites, as well as the first proof-of-principle experiments in quantum sensing. The development of microwave quantum technologies turned out, nonetheless, to be more challenging. Here, we will discuss the challenges regarding the use of microwaves for quantum communication and sensing. Based on this analysis, we propose a roadmap to achieve real-life applications in these fields.},

keywords = {},

pubstate = {published},

tppubtype = {inproceedings}

}