研究者詳細 - 森　貴司

森　貴司（モリ　タカシ）

Mori, Takashi

写真a

所属（所属キャンパス）: 理工学部物理学科（矢上）

職名: 准教授

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Accelerated Decay due to Operator Spreading in Bulk-Dissipated Quantum Systems

Shirai T., Mori T.

Physical Review Letters 133 （ 4 ） 2024年07月

ISSN 00319007

　概要を見る

Markovian open many-body quantum systems display complicated relaxation dynamics. The spectral gap of the Liouvillian characterizes the asymptotic decay rate towards the stationary state, but it has recently been pointed out that the spectral gap does not necessarily determine the overall relaxation time. Our understanding on the relaxation process before the asymptotically long-time regime is still limited. We here present a collective relaxation dynamics of autocorrelation functions in the stationary state. As a key quantity in the analysis, we introduce the instantaneous decay rate, which characterizes the transient relaxation and converges to the conventional asymptotic decay rate in the long-time limit. Our theory predicts that a bulk-dissipated system generically shows an accelerated decay before the asymptotic regime due to the scrambling of quantum information associated with the operator spreading.
- Access to Document (DOI)
Phase crossover induced by dynamical many-body localization in periodically driven long-range spin systems

Rahaman M., Mori T., Roy A.

Physical Review B （Physical Review B） 109 （ 10 ） 2024年03月

ISSN 24699950

　概要を見る

Dynamical many-body freezing occurs in periodic transverse field-driven integrable quantum spin systems. Under freezing conditions, quantum dynamics causes practically infinite hysteresis in the drive response, maintaining its starting value. We find similar resonant freezing in the Lipkin-Meshkov-Glick (LMG) model. In the LMG, the freezing conditions in the driving field suppresses the heating postulated by the eigenstate thermalization hypothesis (ETH) by inducing dynamical many-body localization, or DMBL. This is in contrast to many-body localization (MBL), which requires disorder to suppress ETH. DMBL has been validated by the inverse participation ratio (IPR) of the quasistationary Floquet modes. Similarly to the TFIM, the LMG exhibits high-frequency localization only at freezing points. IPR localization in the LMG deteriorates with an inverse system size law at lower frequencies, which indicates heating to infinite temperature. Furthermore, adiabatically increasing frequency and amplitude from low values raises the Floquet state IPR in the LMG from nearly zero to unity, indicating a phase crossover. This occurrence enables a future technique to construct an MBL engine in clean systems that can be cycled by adjusting drive parameters only.
- Access to Document (DOI)
Liouvillian-gap analysis of open quantum many-body systems in the weak dissipation limit

Mori T.

Physical Review B （Physical Review B） 109 （ 6 ） 2024年02月

ISSN 24699950

　概要を見る

Recent experiments have reported that novel physics emerge in open quantum many-body systems due to an interplay of interactions and dissipation, which stimulate theoretical studies of the many-body Lindblad equation. Although the strong dissipation regime receives considerable interest in this context, this work focuses on the weak bulk dissipation. By examining the spectral property of the many-body Lindblad generator for specific models, we find that its spectral gap shows singularity in the weak dissipation limit when the thermodynamic limit is taken first. Based on analytical arguments and numerical calculations, we conjecture that such a singularity is generic in bulk-dissipated quantum many-body systems and is related to the concept of the Ruelle-Pollicott resonance in chaos theory, which determines the timescale of thermalization of an isolated system. This conjecture suggests that the many-body Lindblad equation in the weak dissipation regime contains nontrivial information on intrinsic properties of a quantum many-body system.
- Access to Document (DOI)
Impact of Measurement Noise on Escaping Saddles in Variational Quantum Algorithms

E Kaminishi, T Mori, M Sugawara, N Yamamoto

arXiv preprint arXiv:2406.09780 2024年
Quantum master equation for many-body systems: Derivation based on the Lieb-Robinson bound

K Shiraishi, M Nakagawa, T Mori, M Ueda

arXiv preprint arXiv:2404.14067 2024年

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研究発表【表示／非表示】

Open-system analysis of thermalization in isolated quantum systems

Takashi Mori

Stability of Quantum Phases in and out of Equilibrium at Various Scales （Bangalore） ,

2024年01月
,
口頭発表（招待・特別）

　概要を見る

Open-system analysis of thermalization in isolated quantum systems
Eigenstate thermalization hypothesis (ETH) explains why an isolated quantum system eventually thermalizes, but it does not tell us much about the timescale of the onset of thermalization. Since the time evolution is unitary, all the eigenvalues of the time evolution operator lie on the unit circle in complex plane. It means that we cannot estimate the thermalization timescale by looking at eigenvalues of the time evolution operator.

In this talk, I argue that an open-system analysis based on the Lindblad quantum master equation gives an estimate of the thermalization timescale of the isolated system [1]. More specifically, we consider a kicked Ising chain under bulk dissipation and investigate the Liouvillian gap, which is the spectral gap of the generator of the dynamics, in the weak dissipation limit. We show that the Liouvillian gap can remain finite even in the weak dissipation limit if the thermodynamic limit is taken first. This finite value of the Liouvillian gap in the weak dissipation limit gives the exponential decay rate of the isolated system. This result is reminiscent of Ruelle-Pollicott resonances in classical chaos. Indeed, we argue that the finite Liouvillian gap in the weak dissipation limit is interpreted as a quantum Ruelle-Pollicott resonance.

For static systems with the time-independent Hamiltonian, a special care is needed. I also explain how we can extract exponential decays hidden in the unitary time evolution of a static system.

[1] T. Mori, arXiv:2311.10304
Connection between decoherence of an open system and thermalization of an isolated system

Takashi Mori

The KITP Program on long-range interacting quantum systems （Santa Barbara） ,

2023年10月

-

2023年12月
,
口頭発表（招待・特別）
- Access to Document (DOI)
Liouvillian gap analysis in the weak dissipation limit

Takashi Mori

Nonequilibrium physics – current trends and future perspectives （Physikzentrum Bad Honnef） ,

2023年08月

-

2023年09月
,
口頭発表（招待・特別）

　概要を見る

Recent experimental advance in cold-atomic physics enables us to implement well- controlled dissipation in quantum many-body systems. It is therefore a fascinating problem to explore generic phenomena arising from the interplay between interactions in many-body systems and dissipation.
Theoretically, the time evolution of a Markovian open quantum system is generated by the Liouvillian of the Lindblad form, whose spectral properties (e.g., the relation between the spectral gap and the relaxation time [1-4]) have been intensively studied up to now. In this talk, I focus on the spectral gap of the Liouvillian (the Liouvillian gap) in the weak dissipation limit. I argue that the Liouvillian gap under a weak bulk dissipation often exhibits a singularity in the thermodynamic limit, which is closely related with the irreversible relaxation that is observed in the chaotic quantum many- body system completely isolated from the environment.

References
[1] M. Žnidarič, Phys. Rev. E 92, 042143 (2015)
[2] T. Mori and T. Shirai, Phys. Rev. Lett. 125, 230604 (2020)
[3] T. Haga, M. Nakagawa, R. Hamazaki, and M. Ueda, Phys. Rev. Lett. 127,
070402 (2021)
[4] T. Mori and T. Shirai, Phys. Rev. Lett. 130, 230404 (2023)
Lindblad equation in open quantum many-body systems

Takashi Mori

Physics of Open Systems and Beyond,

2023年08月
,
口頭発表（招待・特別）
Weak dissipation limit of the Liouvillian gap

Takashi Mori

Numerical Methods in Theoretical Physics 2023 （APCTP, Pohang） ,

2023年07月
,
口頭発表（招待・特別）

　概要を見る

In recent years, open quantum many-body systems have received much attention due to experimental progress that allows us to introduce well-controlled dissipation to quantum many-body systems. A quantum Markov process is generated by the Liouvillian superoperator of the Lindblad form. An important quantity characterizing dissipative quantum dynamics is the Liouvillian gap, i.e., the spectral gap of the Liouvillian. The Liouvillian gap characterizes the asymptotic decay rate in the long-time regime. It was shown that a finite Liouvillian gap in the thermodynamic limit implies exponential decay of correlations in the steady state. As a consequence, a vanishing Liouvillian gap is used as a signature of a dissipative phase transition.
In this talk, I focus on the Liouvillian gap of open quantum many-body systems in the weak dissipation limit. I argue that, counterintuitively, the thermodynamic limit of the Liouvillian gap in a chaotic open Floquet system (i.e., the Hamiltonian of the system of interest periodically depends on time) converges to a nonzero value in the weak dissipation limit. This nontrivial value of the Liouvillian gap is identified as the intrinsic decay rate of the underlying isolated quantum system. I also discuss an analogous result for open static systems, in which the Hamiltonian does not depend on time.