Mori, Takashi

写真a

Affiliation

Faculty of Science and Technology, Department of Physics (Yagami)

Position

Associate Professor

 

Papers 【 Display / hide

  • 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 )  2024.03

    ISSN  24699950

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    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.

  • Liouvillian-gap analysis of open quantum many-body systems in the weak dissipation limit

    Mori T.

    Physical Review B (Physical Review B)  109 ( 6 )  2024.02

    ISSN  24699950

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    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.

  • Quantum and classical Floquet prethermalization

    Ho W.W., Mori T., Abanin D.A., Dalla Torre E.G.

    Annals of Physics (Annals of Physics)  454 2023.07

    ISSN  00034916

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    Time-periodic (Floquet) driving is a powerful way to control the dynamics of complex systems, which can be used to induce a plethora of new physical phenomena. However, when applied to many-body systems, Floquet driving can also cause heating, and lead to a featureless infinite-temperature state, hindering most useful applications. It is therefore important to find mechanisms to suppress such effects. Floquet prethermalization refers to the phenomenon where many-body systems subject to a high-frequency periodic drive avoid heating for very long times, instead tending to transient states that can host interesting physics. Its key signature is a strong parametric suppression of the heating rate as a function of the driving frequency. Here, we review our present understanding of this phenomenon in both quantum and classical systems, and across various models and methods. In particular, we present rigorous theorems underpinning Floquet prethermalization in quantum spin and fermionic lattice systems and extensions to systems with degrees of freedom that have unbounded local dimension. Further, we briefly describe applications to novel nonequilibrium phases of matter, and recent experiments probing prethermalization with quantum simulators. We close by describing the frontiers of Floquet prethermalization beyond strictly time-periodic drives, including time-quasiperiodic driving and long-lived quasi-conserved quantities enabled by large separation of energy scales.

  • Symmetrized Liouvillian Gap in Markovian Open Quantum Systems

    Mori T., Shirai T.

    Physical Review Letters (Physical Review Letters)  130 ( 23 )  2023.06

    ISSN  00319007

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    Markovian open quantum systems display complicated relaxation dynamics. The spectral gap of the Liouvillian characterizes the asymptotic decay rate toward the steady state, but it does not necessarily give a correct estimate of the relaxation time because the crossover time to the asymptotic regime may be too long. We here give a rigorous upper bound on the transient decay of autocorrelation functions in the steady state by introducing the symmetrized Liouvillian gap. The standard Liouvillian gap and the symmetrized one are identical in an equilibrium situation but differ from each other in the absence of the detailed balance condition. It is numerically shown that the symmetrized Liouvillian gap always gives a correct upper bound on the decay of the autocorrelation function, but the standard Liouvillian gap does not.

  • Floquet States in Open Quantum Systems

    Mori T.

    Annual Review of Condensed Matter Physics (Annual Review of Condensed Matter Physics)  14   35 - 56 2023.03

    ISSN  19475454

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    In Floquet engineering, periodic driving is used to realize novel phases of matter that are inaccessible in thermal equilibrium. For this purpose, the Floquet theory provides us a recipe for obtaining a static effective Hamiltonian. Although many existing works have treated closed systems, it is important to consider the effect of dissipation, which is ubiquitous in nature. Understanding the interplay of periodic driving and dissipation is a fundamental problem of nonequilibrium statistical physics that is receiving growing interest because of the fact that experimental advances have allowed us to engineer dissipation in a controllable manner. In this review, we give a detailed exposition on the formalism of quantum master equations for open Floquet systems and highlight recent work investigating whether equilibrium statistical mechanics applies to Floquet states.

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Presentations 【 Display / hide

  • Open-system analysis of thermalization in isolated quantum systems

    Takashi Mori

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

    2024.01

    Oral presentation (invited, special)

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    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) , 

    2023.10
    -
    2023.12

    Oral presentation (invited, special)

  • Liouvillian gap analysis in the weak dissipation limit

    Takashi Mori

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

    2023.08
    -
    2023.09

    Oral presentation (invited, special)

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    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, 

    2023.08

    Oral presentation (invited, special)

  • Weak dissipation limit of the Liouvillian gap

    Takashi Mori

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

    2023.07

    Oral presentation (invited, special)

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    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.

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Awards 【 Display / hide

  • 第22回久保亮五記念賞

    2018.10, 井上科学振興財団

    Type of Award: Award from publisher, newspaper, foundation, etc.

  • 第8回日本物理学会領域11若手奨励賞

    2014.03

    Type of Award: Award from Japanese society, conference, symposium, etc.

 

Courses Taught 【 Display / hide

  • STATISTICAL PHYSICS A

    2024

  • PHYSICS EXERCISE 1

    2024

  • MECHANICS OF FLUID AND ELASTIC BODY

    2024

  • LITERATURE OF PHYSICS

    2024

  • INDEPENDENT STUDY ON FUNDAMENTAL SCIENCE AND TECHNOLOGY

    2024

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