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Stimulated emission of superradiant atoms in waveguide QED

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arXiv 2022年

作者： Asaoka, Rui Gea-Banacloche, Julio Tokunaga, Yuuki Koshino, Kazuki Computer And Data Science Laboratories NTT Corporation Musashino180-8585 Japan Department Of Physics University Of Arkansas FayettevilleAR72701 United States College Of Liberal Arts And Sciences Tokyo Medical And Dental University Ichikawa272-0827 Japan

We investigate the stimulated emission of superradiant atoms coupled to a waveguide induced by a coherentstate photon pulse. We provide an analytical result when a short π pulse is incident, which shows that the atoms emit photons coherently into the output pulse, which remains a coherent state in the short pulse limit. An incident pulse is amplified in phase-preserving manner, where noise is added almost entirely in the phase direction in phase space. This property improves the ratio of intensity signal to noise after the amplification for sufficiently short pulses. This is a unique feature different from general phase-preserving linear amplifiers, where the signal-to-noise ratio deteriorates in the amplification process. We also discuss the dependence of the photon-emission probability on pulse parameters, such as the pulse area and the duration. Copyright © 2022, The Authors. All rights reserved.

关键词： Atoms

Virtual quantum error detection

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Physical Review A 2023年第4期108卷 042426-042426页

作者： Kento Tsubouchi Yasunari Suzuki Yuuki Tokunaga Nobuyuki Yoshioka Suguru Endo Department of Applied Physics University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo 113-8656 Japan NTT Computer and Data Science Laboratories NTT Corporation Musashino Tokyo 180-8585 Japan Theoretical Quantum Physics Laboratory RIKEN Cluster for Pioneering Research Wako-shi Saitama 351-0198 Japan JST PRESTO 4-1-8 Honcho Kawaguchi Saitama 332-0012 Japan

Quantum error correction and quantum error detection necessitate syndrome measurements to detect errors. Performing syndrome measurements for each stabilizer generator can be a significant overhead, considering the fact that the readout fidelity in the current quantum hardware is generally lower than gate fidelity. Here, by generalizing a quantum error mitigation method known as symmetry expansion, we propose a protocol called virtual quantum error detection (VQED). This method virtually allows for evaluating computation results corresponding to postselected quantum states obtained through quantum error detection during circuit execution, without implementing syndrome measurements. Unlike conventional quantum error detection, which requires the implementation of Hadamard test circuits for each stabilizer generator, our VQED protocol can be performed with a constant depth shallow quantum circuit with an ancilla qubit, irrespective of the number of stabilizer generators. Furthermore, for some simple error models, the computation results obtained using VQED are robust against the noise that occurred during the operation of VQED, and our method is fully compatible with other error mitigation schemes, enabling further improvements in computation accuracy and facilitating high-fidelity quantum computing.

关键词： Quantum algorithms Quantum algorithms & computation Quantum error correction Quantum information processing

Experimental demonstration of a high-fidelity virtual two-qubit gate

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arXiv 2023年

作者： Singh, Akhil Pratap Mitarai, Kosuke Suzuki, Yasunari Heya, Kentaro Tabuchi, Yutaka Fujii, Keisuke Nakamura, Yasunobu Department of Applied Physics Graduate School of Engineering The University of Tokyo Bunkyo-ku Tokyo113-8656 Japan Graduate School of Engineering Science Osaka University 1-3 Machikaneyama Toyonaka Osaka560-8531 Japan NTT Computer and Data Science Laboratories NTT Corporation Musashino180-8585 Japan Saitama Wako351-0198 Japan

We experimentally demonstrate a virtual two-qubit gate and characterize it using quantum process tomography (QPT). The virtual two-qubit gate decomposes an actual two-qubit gate into single-qubit unitary gates and projection gates in quantum circuits for expectation-value estimation. We implement projection gates via mid-circuit measurements. The deterministic sampling scheme reduces the number of experimental circuit evaluations required for decomposing a virtual two-qubit gate. We also apply quantum error mitigation to suppress the effect of measurement errors and improve the average gate fidelity of a virtual controlled-Z (CZ) gate to fav = 0.9938 ± 0.0002. Our results highlight a practical approach to implement virtual two-qubit gates with high fidelities, which are useful for simulating quantum circuits using fewer qubits and implementing two-qubit gates on a distant pair of qubits. Copyright © 2023, The Authors. All rights reserved.

关键词： Qubits

Perturbative Quantum Simulation

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Physical Review Letters 2022年第12期129卷 120505-120505页

作者： Jinzhao Sun Suguru Endo Huiping Lin Patrick Hayden Vlatko Vedral Xiao Yuan Center on Frontiers of Computing Studies Peking University Beijing 100871 China Clarendon Laboratory University of Oxford Parks Road Oxford OX1 3PU United Kingdom Quantum Advantage Research Beijing 100080 China NTT Computer & Data Science Laboratories NTT corporation Musashino Tokyo 180-8585 Japan School of Computer Science Peking University Beijing 100871 China Stanford Institute for Theoretical Physics Stanford University Stanford California 94305 USA Centre for Quantum Technologies National University of Singapore Singapore 117543 Singapore

Approximation based on perturbation theory is the foundation for most of the quantitative predictions of quantum mechanics, whether in quantum many-body physics, chemistry, quantum field theory, or other domains. Quantum computing provides an alternative to the perturbation paradigm, yet state-of-the-art quantum processors with tens of noisy qubits are of limited practical utility. Here, we introduce perturbative quantum simulation, which combines the complementary strengths of the two approaches, enabling the solution of large practical quantum problems using limited noisy intermediate-scale quantum hardware. The use of a quantum processor eliminates the need to identify a solvable unperturbed Hamiltonian, while the introduction of perturbative coupling permits the quantum processor to simulate systems larger than the available number of physical qubits. We present an explicit perturbative expansion that mimics the Dyson series expansion and involves only local unitary operations, and show its optimality over other expansions under certain conditions. We numerically benchmark the method for interacting bosons, fermions, and quantum spins in different topologies, and study different physical phenomena, such as information propagation, charge-spin separation, and magnetism, on systems of up to 48 qubits only using an 8+1 qubit quantum hardware. We demonstrate our scheme on the IBM quantum cloud, verifying its noise robustness and illustrating its potential for benchmarking large quantum processors with smaller ones.

关键词： Quantum algorithms Quantum computation Quantum simulation

Fundamental limits of quantum error mitigation

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arXiv 2021年

作者： Takagi, Ryuji Endo, Suguru Minagawa, Shintaro Gu, Mile Nanyang Quantum Hub School of Physical and Mathematical Sciences Nanyang Technological University 637371 Singapore NTT Computer and Data Science Laboratories NTT Corporation Musashino Tokyo180-8585 Japan Graduate School of Informatics Nagoya University Chikusa-ku Nagoya464-8601 Japan Centre for Quantum Technologies National University of Singapore 3 Science Drive 2 117543 Singapore

The inevitable accumulation of errors in near-future quantum devices represents a key obstacle in delivering practical quantum advantages, motivating the development of various quantum error-mitigation methods. Here, we derive fundamental bounds concerning how error-mitigation algorithms can reduce the computation error as a function of their sampling overhead. Our bounds place universal performance limits on a general error-mitigation protocol class. We use them to show (1) that the sampling overhead that ensures a certain computational accuracy for mitigating local depolarizing noise in layered circuits scales exponentially with the circuit depth for general error-mitigation protocols and (2) the optimality of probabilistic error cancellation among a wide class of strategies in mitigating the local dephasing noise on an arbitrary number of qubits. Our results provide a means to identify when a given quantum error-mitigation strategy is optimal and when there is potential room for improvement. © 2021, CC BY.

关键词： Quantum noise

Circuit designs for practical-scale fault-tolerant quantum computing

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Circuit designs for practical-scale fault-tolerant quantum c...

Symposium on VLSI Technology

作者： Yasunari Suzuki Yosuke Ueno Wang Liao Masamitsu Tanaka Teruo Tanimoto NTT Computer & Data Science Laboratories Musashino Tokyo Japan JST RESTO Kawaguchi Saitama Japan Keio University Yokohama Kanagawa Japan The University of Tokyo Bunkyo-ku Tokyo Japan Kyushu University Fukuoka Japan Nagoya University Nagoya Aichi Japan

To demonstrate reliable and scalable quantum computation, we need quantum error correction to reduce its error rates. One of the most challenging parts of implementing quantum error correction is to design error-decoding units, which estimate errors during computation. We estimate the required performances of error-decoding units to run practical-scale quantum algorithms and discuss the directions to satisfy them.

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Unitary-transformed projective squeezing: applications for circuit-knitting and state-preparation of non-Gaussian states

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arXiv 2024年

作者： Anai, Keitaro Suzuki, Yasunari Tokunaga, Yuuki Matsuzaki, Yuichiro Takeda, Shuntaro Endo, Suguru Department of Applied Physics University of Tokyo 7-3-1 Hongo Bunkyo-ku Tokyo113-8656 Japan NTT Computer and Data Science Laboratories NTT Corporation Musashino Tokyo180-8585 Japan Department of Electrical Electronic and Communication Engineering Faculty of Science and Engineering Chuo University 1-13-27 Kasuga Bunkyo-ku Tokyo112-8551 Japan

Continuous-variable (CV) quantum computing is a promising candidate for quantum computation because it can utilize infinite-dimensional Hilbert space even with one mode and efficiently handle continuous values. Although the photonic platforms have been considered as a leading platform for CV computation, hybrid systems comprising qubits and bosonic modes, e.g., superconducting hardware, have shown significant developments because they can prepare non-Gaussian state by utilizing the nonlinear interaction between the qubits and the bosonic modes. However, the experimentally realizable size of hybrid hardware is currently restricted. Moreover, the fidelity of the non-Gaussian state is still limited. In this work, by developing the projective squeezing method, we establish the formalism for projecting quantum states onto the states that are unitary-transformed from the squeezed vacuum at the expense of the sampling cost. Based on this formalism, we propose methods for simulating larger quantum devices and projecting the state onto one of the typical non-Gaussian states, the cubic phase state, with a higher squeezing level and higher nonlinearity. For practical implementation, we can apply the smeared projector either by linear-combination-of-unitaries or virtual quantum error detection algorithms by leveraging the interactions in hybrid systems of qubits and bosonic modes. We numerically verify the performance of our methods and also show that the effect of photon-loss errors can be suppressed due to projection. Copyright © 2024, The Authors. All rights reserved.

关键词： Qubits

Efficient Simulation of Leakage Errors in Quantum Error Correcting Codes Using Tensor Network Methods

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arXiv 2023年

作者： Manabe, Hidetaka Suzuki, Yasunari Darmawan, Andrew S. Graduate School of Engineering Science Osaka University 1-3 Machikaneyama Toyonaka Osaka560-8531 Japan NTT Computer and Data Science Laboratories NTT Corporation Musashino180-8585 Japan JST PRESTO 4-1-8 Honcho Saitama Kawaguchi332-0012 Japan Kyoto University Kitashirakawa Oiwakecho Sakyo-ku Kyoto606-8502 Japan

Leakage errors, in which a qubit is excited to a level outside the qubit subspace, represent a significant obstacle in the development of robust quantum computers. We present a computationally efficient simulation methodology for studying leakage errors in quantum error correcting codes (QECCs) using tensor network methods, specifically Matrix Product States (MPS). Our approach enables the simulation of various leakage processes, including thermal noise and coherent errors, without approximations (such as the Pauli twirling approximation) that can lead to errors in the estimation of the logical error rate. We apply our method to two QECCs: the one-dimensional (1D) repetition code and a thin 3 × d surface code. By leveraging the small amount of entanglement generated during the error correction process, we are able to study large systems, up to a few hundred qudits, over many code cycles. We consider a realistic noise model of leakage relevant to superconducting qubits to evaluate code performance and a variety of leakage removal strategies. Our numerical results suggest that appropriate leakage removal is crucial, especially when the code distance is large. Copyright © 2023, The Authors. All rights reserved.

关键词： Thermal noise

Boltzmann machine learning with a variational quantum algorithm

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arXiv 2020年

作者： Shingu, Yuta Seki, Yuya Watabe, Shohei Endo, Suguru Matsuzaki, Yuichiro Kawabata, Shiro Nikuni, Tetsuro Hakoshima, Hideaki Department of Physics Faculty of Science Division I Tokyo University of Science Shinjuku Tokyo162-8601 Japan 1-1-1 Umezono Ibaraki Tsukuba305-8568 Japan Department of Materials University of Oxford Parks Road OxfordOX1 3PH United Kingdom NTT Computer Data Science Laboratories NTT Corporation Musashino Tokyo180-8585 Japan

A Boltzmann machine is a powerful tool for modeling probability distributions that govern the training data. A thermal equilibrium state is typically used for the Boltzmann machine learning to obtain a suitable probability distribution. The Boltzmann machine learning consists of calculating the gradient of the loss function given in terms of the thermal average, which is the most time consuming procedure. Here, we propose a method to implement the Boltzmann machine learning by using Noisy Intermediate-Scale Quantum (NISQ) devices. We prepare an initial pure state that contains all possible computational basis states with the same amplitude, and apply a variational imaginary time simulation. Readout of the state after the evolution in the computational basis approximates the probability distribution of the thermal equilibrium state that is used for the Boltzmann machine learning. We perform the numerical simulations of our scheme and confirm that the Boltzmann machine learning works well. Our scheme leads to a significant step toward an efficient machine learning using quantum hardware. Copyright © 2020, The Authors. All rights reserved.

关键词： Probability distributions