- 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.
- 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.
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)
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.
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, 127 (10), pp. 100503, 2021.
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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.
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)
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.
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)
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.
Ding, Y.; Martín-Guerrero, J. D.; Song, Y.; Magdalena-Benedito, R.; Chen, X.
Active Learning for the Optimal Design of Multinomial Classification in Physics (Miscellaneous)
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% samples labeled. We reckon that active-learning-inspired physics experiments will remarkably save budget without loss of accuracy.
Girard, J. P.; Liu, W.; Kokkoniemi, R.; Visakorpi, E.; Govenius, J.; Möttönen, M.
Cryogenic power sensor enabling broad-band and traceable measurements (Miscellaneous)
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.
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, 16 (2), pp. 024044, 2021.
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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.
Chen, Q. -M.; Fesquet, F.; Honasoge, K. E.; Kronowetter, F.; 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 (Miscellaneous)
We propose a tunable coupler consisting of N off-resonant and fixed-frequency qubits that can tune and even amplify the effective interaction between two general circuit components. 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 in the system. 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 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.
García-Molina, P.; Martin, A.; Sanz, M.
Noise in Digital and Digital-Analog Quantum Computation (Miscellaneous)
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.
Wang, R.; Hernani-Morales, C.; Martín-Guerrero, J. D.; Solano, E.; Albarrán-Arriagada, F.
Quantum Pattern Recognition in Photonic Circuits (Miscellaneous)
We propose a machine learning method to characterize photonic states via a simple optical circuit and the data processing of photon number distributions 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 a supervised learning algorithm to predict the degree of entanglement in the two-mode state and to perform the full tomography of one photonic mode, obtaining good accuracy and an $r$-factor performance of our algorithm $r > 0.75$.
Kumar, S.; Cárdenas-López, F. A.; Hegade, N. N.; Chen, X.; Albarrán-Arriagada, F.; Solano, E.; Barrios, G. A.
Entangled Quantum Memristors (Miscellaneous)
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 their 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 NISQ technologies.
Chandarana, P.; Hegade, N. N.; Paul, Koushik; Albarrán-Arriagada, F.; Solano, Enrique; Campo, A.; Chen, Xi
Digitized-counterdiabatic quantum approximate optimization algorithm (Miscellaneous)
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 QAOA is an ansatz-dependent algorithm, there is always a need to design ansatz for better optimization. To this end, we propose a digitized version of 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-counterdiabatic QAOA to Ising models, classical optimization problems, and the P-spin model, demonstrating that it outperforms standard QAOA in all cases we study.
Tobalina, A.; Munuera-Javaloy, C.; Torrontegui, E.; Muga, J. G.; Casanova, J.
Tailored Ion Beam for Precise Color Center Creation (Miscellaneous)
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.
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, 103 (6), pp. 062201, 2021.
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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.
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, 103 (22), pp. 224525, 2021.
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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.
Miranda, E. R.; Venkatesh, S.; Hernani-Morales, C.; Lamata, L.; Martín-Guerrero, J. D.; Solano, E.
Quantum Brain Networks: a Perspective (Miscellaneous)
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.
Barraza, N.; Pan, C. -Y.; Lamata, L.; Solano, E.; Albarrán-Arriagada, F.
Adaptive Random Quantum Eigensolver (Miscellaneous)
We propose an adaptive random quantum algorithm to obtain an optimized eigensolver. The changes in the involved matrices follow bio-inspired evolutionary mutations which are 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 (NISQ) computers.
Costa, N. F.; Omar, Y.; Sultanov, A.; Paraoanu, G. S.
In: EPJ Quantum Technology, 8 (1), 2021.
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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.
Pan, C. -Y.; Hao, M.; Barraza, N.; Solano, E.; Albarrán-Arriagada, F.
Experimental semi-autonomous eigensolver using reinforcement learning (Journal Article)
In: Scientific Reports, 11 (1), 2021.
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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.
Yu, J.; Cárdenas-López, F. A.; Andersen, C. K.; Solano, E.; Parra-Rodriguez, A.
Charge qubits in the ultrastrong coupling regime (Miscellaneous)
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.
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, 2 (2), pp. 020323, 2021.
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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.
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, 2 (2), pp. 020328, 2021.
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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.
Hegade, N. N.; Paul, K.; Albarrán-Arriagada, F.; Chen, X.; Solano, E.
Digitized-adiabatic Quantum Factorization (Miscellaneous)
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.
Ban, Y.; Echanobe, J.; Torrontegui, E.; Casanova, J.
Developments of Neural Networks in Quantum Physics (Miscellaneous)
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, the application of classical neural network 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 optimized with the aid 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.
Ban, Yue; Torrontegui, E.; Casanova, J.
Quantum neural networks with multi-qubit potentials (Miscellaneous)
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.
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, 103 (4), pp. l040401, 2021.
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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.
Munuera-Javaloy, C.; Puebla, R.; Casanova, J.
Dynamical decoupling methods in nanoscale NMR (Journal Article)
In: Europhysics Letters, 134 , pp. 30001, 2021.
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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.
Unconditional Microwave Quantum Teleportation of Gaussian States in Lossy Environments (Miscellaneous)
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.
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 (Miscellaneous)
The modern field of quantum communication thrives on promise to deliver efficient and unconditionally secure ways to exchange information by exploiting quantum laws of physics. Here, quantum teleportation stands out as an exemplary protocol allowing for the disembodied and safe transfer of unknown quantum states using quantum entanglement and classical communication as resources. The experimental feasibility of quantum teleportation with propagating waves, relevant to communication scenarios, has been demonstrated in various physical settings. However, an analogous implementation of quantum teleportation in the microwave domain was missing so far. At the same time, recent breakthroughs in quantum computation with superconducting circuits have triggered a demand for quantum communication between spatially separated superconducting processors operated at microwave frequencies. Here, we demonstrate a realization of deterministic quantum teleportation of coherent microwave states by exploiting two-mode squeezing and analog feedforward over macroscopic distances $d = 42,$cm. We achieve teleportation fidelities $F = 0.689 pm 0.004$ exceeding the no-cloning $F_mathrmnc = 2/3$ threshold for coherent states with an average photon number of up to $n_mathrmđ = 1.1$. Our results provide a key ingredient for the teleportation-based quantum gate for modular quantum computing with superconducting circuits and establish a solid foundation for future microwave quantum local area networks.
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: Physical Review B, 103 (9), pp. 094515, 2021.
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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.
Yu, J.; Retamal, J. C.; Sanz, M.; Solano, E.; Albarrán-Arriagada, F.
Superconducting Circuit Architecture for Digital-Analog Quantum Computing (Miscellaneous)
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 $2<ell łeq h$), using only $2(2ell+1)^2+24$ analog blocks. The proposed architecture design is feasible in current experimental setups for quantum computing with superconducting circuits, opening the door to useful quantum advantage with fewer resources.
Céleri, L. C.; Huerga, D.; Albarrán-Arriagada, F.; Solano, E.; Sanz, M.
Digital-analog quantum simulation of fermionic models (Miscellaneous)
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.
Ban, Y.; Chen, X.; Torrontegui, E.; Solano, E.; Casanova, J.
Speeding up quantum perceptron via shortcuts to adiabaticity (Journal Article)
In: Scientific Reports, 11 (1), 2021.
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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.
Pires, D. P.; Modi, K.; Céleri, L. C.
In: Physical Review E, 103 (3), pp. 032105, 2021.
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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.
Dong, L.; Arrazola, I.; Chen, X.; Casanova, J.
In: Physical Review Applied, 15 (3), pp. 034055, 2021.
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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.
Bugalho, L.; Coutinho, B. C.; Omar, Y.
Distributing Multipartite Entanglement over Noisy Quantum Networks (Miscellaneous)
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.
Gatti, G.; Huerga, D.; Solano, E.; Sanz, M.
Random access codes via quantum contextual redundancy (Miscellaneous)
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.
Coutinho, B. C.; Munro, W. J.; Nemoto, K.; Omar, Y.
Robustness of Noisy Quantum Networks (Miscellaneous)
Quantum networks are a new paradigm of complex networks, allowing 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 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. In particular, our results indicate that a scale-free topology is a crucial design principle to establish a robust large-scale quantum internet.
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, 15 (2), pp. 024038, 2021.
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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.
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, 3 (1), pp. 013167, 2021.
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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.
Barrios, G. A.; Albarrán-Arriagada, F.; Peña, F. J.; Solano, E.; Retamal, J. C.
Light-matter quantum Otto engine in finite time (Miscellaneous)
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.
Ding, Y.; Chen, X.; Lamata, L.; Solano, E.; Sanz, M.
In: SN Computer Science, 2 (2), 2021.
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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.
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 (Miscellaneous)
Robust and high-precision quantum control is crucial but challenging for scalable quantum computation and quantum information processing. Traditional adiabatic control suffers severe limitations on gate performance imposed by environmentally induced noise because of a quantum system's limited coherence time. In this work, we experimentally demonstrate an alternative approach to quantum control based on deep reinforcement learning (DRL) on a trapped $^171mathrmYb^+$ ion. In particular, we find that DRL leads to fast and robust digital quantum operations with running time bounded by shortcuts to adiabaticity (STA). Besides, we demonstrate that DRL's robustness against both Rabi and detuning errors can be achieved simultaneously without any input from STA. Our experiments reveal a general framework of digital quantum control, leading to a promising enhancement in quantum information processing.
Gonzalez-Conde, J.; Rodríguez-Rozas, Á.; Solano, E.; Sanz, M.
Pricing Financial Derivatives with Exponential Quantum Speedup (Miscellaneous)
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%.
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 (Miscellaneous)
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 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 noises. 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.
Pires, D.; Bargassa, P.; Seixas, J.; Omar, Y.
A Digital Quantum Algorithm for Jet Clustering in High-Energy Physics (Journal Article)
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.
Pires, D.; Omar, Y.; Seixas, J.
Adiabatic Quantum Algorithm for Multijet Clustering in High Energy Physics (Miscellaneous)
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.
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, 384 (36), pp. 126933, 2020.
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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.
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 of parametric amplification (Miscellaneous)
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 1 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.
Munuera-Javaloy, C.; Ban, Y.; Chen, X.; Casanova, J.
Robust Detection of High-Frequency Signals at the Nanoscale (Journal Article)
In: Physical Review Applied, 14 (5), pp. 054054, 2020.
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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.