研究者詳細 - 山本　直樹

論文 - 山本　直樹

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Towards Single-Input Single-Output Nonlinear System Identification and Signal Processing on Near-Term Quantum Computers

Chen J., Nurdin H.I., Yamamoto N.

Proceedings of the IEEE Conference on Decision and Control （Proceedings of the IEEE Conference on Decision and Control） 2019-December 401 - 406 2019年12月

ISSN 07431546

Based on a recently developed theory, we propose a realization of a single-input single-output (SISO) quantum reservoir computer on a near-term quantum computer for approximating SISO fading memory maps. Such a quantum reservoir computer can be of interest for applications such as nonlinear system identification and nonlinear signal processing (e.g., speech processing), opening an avenue for early control-oriented applications of near-term quantum computers. We detail an implementation of the quantum reservoir computer on a cloud-based 20 qubit IBM quantum computer and report on simulation results for the proposed scheme on an IBM Qiskit quantum computer simulator for this machine.

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Semiclassical phase reduction theory for quantum synchronization

Kato Y., Yamamoto N., Nakao H.

Physical Review Research （Physical Review Research） 1 （ 3 ） 2019年10月

ISSN 26431564

We develop a general theoretical framework of semiclassical phase reduction for analyzing synchronization of quantum limit-cycle oscillators. The dynamics of quantum dissipative systems exhibiting limit-cycle oscillations are reduced to a simple, one-dimensional classical stochastic differential equation approximately describing the phase dynamics of the system under the semiclassical approximation. The density matrix and power spectrum of the original quantum system can be approximately reconstructed from the reduced phase equation. The developed framework enables us to analyze synchronization dynamics of quantum limit-cycle oscillators using the standard methods for classical limit-cycle oscillators in a quantitative way. As an example, we analyze synchronization of a quantum van der Pol oscillator under harmonic driving and squeezing, including the case that the squeezing is strong and the oscillation is asymmetric. The developed framework provides insights into the relation between quantum and classical synchronization and will facilitate systematic analysis and control of quantum nonlinear oscillators.

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Control limit on quantum state preparation under decoherence

Kobayashi K., Yamamoto N.

Physical Review A （Physical Review A） 99 （ 5 ） 2019年05月

ISSN 24699926

Quantum information technologies require careful control for generating and preserving a desired target quantum state. The biggest practical obstacle is of course decoherence. Therefore, the reachability analysis, which in our scenario aims to estimate the distance between the controlled state under decoherence and the target state, is of great importance to evaluate the realistic performance of those technologies. This paper presents a lower bound of the fidelity-based distance for a general open Markovian quantum system driven by the decoherence process and several types of control including feedback. The lower bound is straightforward to calculate and can be used as a guide for choosing the target state, as demonstrated in some examples. Moreover, the lower bound is applied to derive a theoretical limit in some quantum metrology problems based on a large-size atomic ensemble under control and decoherence.

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Generating robust entanglement via quantum feedback

Yoshimura K., Yamamoto N.

Journal of Physics B: Atomic, Molecular and Optical Physics （Journal of Physics B: Atomic, Molecular and Optical Physics） 52 （ 5 ） 2019年02月

ISSN 09534075

Generating entangled states is one of the most important tasks in quantum information technology. However, in reality any entanglement generator must contain some characteristic uncertainty, and as a result the produced entangled state becomes an undesirable mixed state. This paper develops a coherent feedback control scheme that suppresses the characteristic uncertainty of a typical entanglement generator (the non-degenerate optical parametric oscillator) for producing robust Gaussian entangled states. In particular, we examine a two-mode squeezed state and Gaussian four-mode cluster states to demonstrate the effectiveness of the proposed control method.

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Sensitivity Analysis of Cascaded Quantum Feedback Amplifier

Yokotera Y., Yamamoto N.

IEEE Control Systems Letters （IEEE Control Systems Letters） 3 （ 1 ） 156 - 161 2019年01月

Quantum amplifier is an essential device in quantum information processing. As in the classical (non-quantum) case, its characteristic uncertainty needs to be suppressed by feedback, and in fact such a control theory for a single quantum amplifier has recently been developed. This letter extends this result to the case of cascaded quantum amplifier. In particular, we consider two types of structures: the case where controlled amplifiers are connected in series, and the case where a single feedback control is applied to the cascaded amplifier. Then, we prove that the latter is better in the sense of sensitivity to the uncertainty. A detailed numerical simulation is given to show actual performance of these two feedback schemes.

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A systems theory approach to the synthesis of non-reciprocal phase-insensitive quantum amplifiers

Petersen I.R., James M.R., Ugrinovskii V., Yamamoto N.

AIAA Scitech 2019 Forum （AIAA Scitech 2019 Forum） 2019年

We present a synthesis procedure for constructing a non-reciprocal phase-insensitive quantum amplifier using quantum optics. An expression is obtained for the corresponding level of noise by this amplifier. This noise level is greater than the level required in a previous paper which did not impose the non-reciprocal requirement. The synthesis procedure uses a singularly perturbed quantum system and leads to an amplifier involving three squeezers and a number of beamsplitters.

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A Systems Theory Approach to the Synthesis of Minimum Noise Phase-Insensitive Quantum Amplifiers

Petersen I.R., James M.R., Ugrinovskii V., Yamamoto N.

2018 European Control Conference, ECC 2018 （2018 European Control Conference, ECC 2018） 3185 - 3190 2018年11月

We present a systems theory approach to the proof of a result bounding the required level of added quantum noise in a phase-insensitive quantum amplifier. We also present a synthesis procedure for constructing a quantum optical phase-insensitive quantum amplifier which adds the minimum level of quantum noise and achieves a required gain and bandwidth. This synthesis procedure is based on a singularly perturbed quantum system and leads to an amplifier involving two squeezers and two beamsplitters.

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Replacing measurement feedback with coherent feedback for quantum state preparation

Kashiwamura Y., Yamamoto N.

Physical Review A （Physical Review A） 97 （ 6 ） 2018年06月

ISSN 24699926

Measurement feedback is a versatile and powerful tool, although its performance is limited by several practical imperfections resulting from classical components. This paper shows that, for some typical quantum feedback control problems for state preparation (stabilization of a qubit or a qutrit, spin squeezing, and Fock state generation), the classical feedback operation can be replaced by a fully quantum one such that the state autonomously dissipates into the target or a state close to the target. The main common feature of the proposed quantum operation, which is called coherent feedback, is that it is composed of a series of dispersive and dissipative couplings inspired by the corresponding measurement feedback scheme.

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Cascade and locally dissipative realizations of linear quantum systems for pure Gaussian state covariance assignment

Ma S., Woolley M.J., Petersen I.R., Yamamoto N.

Automatica （Automatica） 90 263 - 270 2018年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0005-1098

This paper presents two realizations of linear quantum systems for covariance assignment corresponding to pure Gaussian states. The first one is called a cascade realization; given any covariance matrix corresponding to a pure Gaussian state, we can construct a cascaded quantum system generating that state. The second one is called a locally dissipative realization; given a covariance matrix corresponding to a pure Gaussian state, if it satisfies certain conditions, we can construct a linear quantum system that has only local interactions with its environment and achieves the assigned covariance matrix. Both realizations are illustrated by examples from quantum optics.

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Dissipative-dissipative control strategy for quantum coherent feedback

Y. Kashiwamura and N. Yamamoto

Proceedings of IFAC World Congress 11760 - 11763 2017年12月

研究論文（国際会議プロシーディングス）, 共著, 査読有り

Entanglement-assisted quantum feedback control

N. Yamamoto and T. Mikami

Quantum Information Processing 16 （ 7 ） 179 2017年07月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1570-0755

The main advantage of quantum metrology relies on the effective use of entanglement, which indeed allows us to achieve strictly better estimation performance over the standard quantum limit. In this paper, we propose an analogous method utilizing entanglement for the purpose of feedback control. The system considered is a general linear dynamical quantum system, where the control goal can be systematically formulated as a linear quadratic Gaussian control problem based on the quantum Kalman filtering method; in this setting, an entangled input probe field is effectively used to reduce the estimation error and accordingly the control cost function. In particular, we show that, in the problem of cooling an opto-mechanical oscillator, the entanglement-assisted feedback control can lower the stationary occupation number of the oscillator below the limit attainable by the controller with a coherent probe field and furthermore beats the controller with an optimized squeezed probe field.

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Pure Gaussian states from quantum harmonic oscillator chains with a single local dissipative process

S. Ma, M. J. Woolley, I. R. Petersen, and N. Yamamoto

Journal of Physics A: Mathematical and Theoretical 50 （ 13 ） 2017年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 1751-8113

We study the preparation of entangled pure Gaussian states via reservoir engineering. In particular, we consider a chain consisting of (2N + 1) quantum harmonic oscillators where the central oscillator of the chain is coupled to a single reservoir. We then completely parametrize the class of (2N + 1)-mode pure Gaussian states that can be prepared by this type of quantum harmonic oscillator chain. This parametrization allows us to determine the steady-state entanglement properties of such quantum harmonic oscillator chains.

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Optimal control for perfect state transfer in linear quantum memory

H. Nakao and N. Yamamoto

Journal of Physics B: Atomic, Molecular and Optical Physics 50 （ 6 ） 2017年02月

研究論文（学術雑誌）, 共著, 査読有り, ISSN 0953-4075

A quantum memory is a system that enables transfer, storage, and retrieval of optical quantum states by ON/OFF switching of the control signal in each stage of the memory. In particular, it is known that, for perfect transfer of a single-photon state, appropriate shaping of the input pulse is required. However, in general, such a desirable pulse shape has a complicated form, which would be hard to generate in practice. In this paper, for a wide class of linear quantum memory systems, we develop a method that reduces the complexity of the input pulse shape of a single photon while maintaining the perfect state transfer. The key idea is twofold; (i) the control signal is allowed to vary continuously in time to introduce an additional degree of freedom, and then (ii) an optimal control problem is formulated to design a simple-formed input pulse and the corresponding control signal. Numerical simulations are conducted for Λ-type atomic media and networked atomic ensembles, to show the effectiveness of the proposed method.

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Quantum back-action evasion via coherent feedback control

Y. Yokotera and N. Yamamoto

Proceedings of 55th IEEE CDC 2539 - 2542 2016年12月

研究論文（国際会議プロシーディングス）, 共著, 査読有り

Engineering a detector for sensing an extremely tiny signal such as the gravitational-wave force is a very important subject in quantum physics. A major obstacle towards this goal is that, in the standard detection scheme, the measurement noise is lower bounded by the so-called standard quantum limit (SQL), which is originated from the intrinsic measurement back-action. Hence the detector system has to be carefully engineered so that it evades the back-action and eventually beats the SQL; feedback control is, of course, expected to be a very effective method to achieve this goal. In this paper, we show that this engineering problem can be formulated and solved within the framework of well-developed geometric control theory, for a specific opto-mechanical system.

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Pure Gaussian quantum states from passive Hamiltonians and an active local dissipative process

S. Ma, M. J. Woolley, I. R. Petersen, and N. Yamamoto

Proceedings of 55th IEEE CDC 2519 - 2522 2016年12月

研究論文（国際会議プロシーディングス）, 共著, 査読有り

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Perfect single device absorber of arbitrary traveling single photon fields with a tunable coupling parameter

H. I. Nurdin, M. R. James, and N. Yamamoto

Proceedings of 55th IEEE CDC 2513 - 2518 2016年12月

研究論文（国際会議プロシーディングス）, 共著, 査読有り

We describe an ideal tuneable input-output open quantum system, realizable as either a single qubit or single resonator system, which can perfectly absorb traveling single photon states of arbitrary temporal profiles. That is, the system has a time-dependent coupling parameter to an external field that can be modulated according to the temporal profile of a single photon. We obtain an explicit analytical form for the modulation function that enables perfect absorption of single photon fields with arbitrary temporal profiles by solving the QSDE describing the system being driven by a traveling single photon field. However, a practical limit on the coupling strength prohibits a faithful physical realization of the absorber, and a numerical example is included to illustrate the absorber's performance under constrained coupling strengths.