研究室メンバーによるレギュラー・セミナー、 および、他研究室・機関からのゲスト・スピーカーのセミナーを開催しています。 レギュラー・セミナーは下記要領で行います。

• 夏学期： 古典的論文のレヴューを行います。配布資料は英語、発表は英語もしくは日本語です。
  
• 冬学期： 研究結果の紹介もしくは研究に関係したレビューを行います。発表は英語です。

Access map：理学部1号館 (Faculty of Science Bldg. 1) / 理学部4号館 (Faculty of Science Bldg. 4)

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夏学期 / Summer semester (April-July 2024)

<Regular seminars (review of seminal papers)>
    木曜日13時から理学部１号館414号室で行います(普段と場所or時間の異なる場合は赤字で示します)。
    Each seminar starts from 13:00, Thursday @ #414, Faculty of Science Bldg. 1 (unless otherwise indicated).

<Schedule>
Apr. 18 Hokkyo
    "The Classical Limit of Quantum Spin Systems"
    E. H. Lieb, Commun.Math. Phys. 31, 327-340 (1973).

Apr. 25 (No seminar)

May. 2 Shiraishi
    "Theory of Thermal Transport Coefficients"
    J. M. Luttinger, Phys. Rev. 135, A1505 (1964).
    "A Quantum-statistical Theory of Transport Processes"
    H. Mori, J. Phys. Soc. Jpn. 11 1029 (1956).

May. 9 Li
    "Exact Analysis of an Interacting Bose Gas. I. The General Solution and the Ground State"
    E. H. Lieb and W. Liniger, Phys. Rev. 130, 1605 (1963).
    "Exact Analysis of an Interacting Bose Gas. II. The Excitation Spectrum"
    E. H. Lieb, Phys. Rev. 130, 1616 (1963).

May. 16 (No seminar)

May. 23 (No seminar)

May. 30 Sakamoto(starting from 3pm)
    "Quantum cryptography: Public key distribution and coin tossing"
    C.H. Bennett, G. Brassard, Proceedings of IEEE International Conference on Computers Systems and Signal Processing, Bangalore India, 175 (1984).

Jun. 6 Sugiura
    "The renormalization group and the ϵ expansion"
    Kenneth G. Wilson and J. Kogut, Physics Reports 12, 2, 75-199 (1974).

Jun. 13 Ishii
    "'Luttinger liquid theory' of one-dimensional quantum fluids. I. Properties of the Luttinger model and their extension to the general 1D interacting spinless Fermi gas"
    F. D. M. Haldane, Journal of Physics C: Solid State Physics, 14(19), 2585 (1981).

Jun. 20 Ogawa (B4 student)
    "The Uncertainty Relation Between Energy and Time in Non-relativistic Quantum Mechanics"
    L. Mandelstam and Ig. Tamm, J. Phys. USSR 9, 249-254 (1945).
    "The maximum speed of dynamical evolution"
    Margolus Norman and Lev B. Levitin, Physica D: Nonlinear Phenomena 120.1-2 (1998): 188-195.
    "New form of the time-energy uncertainty relation"
    Eric A. Gislason, Nora H. Sabelli, and John W. Wood, Phys. Rev. A 31, 2078 (1985).

Jun. 27 (No seminar)

Jul. 4 Kandabashi (B4 student)
    "Nonequilibrium Equality for Free Energy Differences"
    C. Jarzynski, Phys. Rev. Lett. 78(14) 2690 (1997).

Jul. 11 Oyaizu (@233 → changed to 913)
    "Classical Time Crystals"
    Alfred Shapere and Frank Wilczek, Phys. Rev. Lett. 109, 160402 (2012).
    "Quantum Time Crystals"
    Frank Wilczek, Phys. Rev. Lett. 109, 160401 (2012).

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冬学期 / Winter semester (October 2024-January 2025)

   木曜日13時から理学部１号館913号室で行います(普段と場所or時間の異なる場合は赤字で示します)。
    Each seminar starts from 13:00, Thursday @ #913, Faculty of Science Bldg. 1 (unless otherwise indicated).
<Schedule>
10/3 (No seminar)
10/10 Kai Li/Hongchao Li (@414)
10/17 Sakamoto
10/24 Hokkyo
10/31 Ishii
11/7 (No seminar)
11/14 (No seminar)
11/21 (No seminar)
11/28 Nakagawa
12/5 (No seminar)
12/12 Shiraishi
12/19 Sugiura
12/26 (No seminar)
1/9 Oyaizu (@207)
1/16 Nohara / Yamada (@1320 from 12:30)

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speaker	Kai Li
title	Probing non-Hermitian eigenenergies
abstract	While non-Hermitian Hamiltonians have been experimentally realized in cold atom systems, it remains an outstanding open question of how to experimentally measure their complex energy spectra in momentum space for a realistic system with boundaries. The existence of non-Hermitian skin effects may make the question even more difficult to address given the fact that energy spectra for a system with open boundaries are dramatically different from those in momentum space; the fact may even lead to the notion that momentum-space band structures are not experimentally accessible for a system with open boundaries. In this study [1], we generalize the widely used radio-frequency spectroscopy to measure both real and imaginary parts of complex energy spectra of a non-Hermitian quantum system for either bosonic or fermionic atoms. By weakly coupling the energy levels of a non-Hermitian system to auxiliary energy levels, we theoretically derive a formula showing that the decay of atoms on the auxiliary energy levels reflects the real and imaginary parts of energy spectra in momentum space. We prove that measurement outcomes are independent of boundary conditions in the thermodynamic limit, providing strong evidence that the energy spectrum in momentum space is experimentally measurable. We also discuss whether the spectrum under open boundary conditions can be measured when skin effects exist and how the interaction in the non-Hermitian system affects the measurement results. [1] K. Li and Y. Xu, Phys. Rev. Lett. 129, 093001 (2022).

speaker	Hongchao Li
title	Dissipative Superfluidity in a Molecular Bose-Einstein Condensate
abstract	Quantum gases of dipolar molecules, which serve as a platform to realize clean and controllable long-range interacting systems, have received considerable attention in the fields of many-body physics and quantum simulation. However, heteronuclear molecules inevitably suffer the two-body loss due to chemical reactions, which is particularly serious for bosonic molecules. Recently, with the development of the microwave shielding the first experimental realization of a BEC of heteronuclear molecules has been reported. Thus, it is of fundamental interest to understand whether or not superfluidity exists under two-body loss in such BECs, since dissipation may deteriorate the phase coherence of a superfluid. In this study [1], we develop superfluid transport theory for a dissipative BEC to show that a weak uniform two-body loss can induce phase rigidity, leading to superfluid transport of bosons even without repulsive interparticle interactions. We also show a generalized f-sum rule for a dissipative superfluid as a consequence of weak U(1) symmetry. Finally, we demonstrate that dissipation enhances the stability of a molecular BEC with dipolar interactions. [1]: H. Li, X. Yu, M. Nakagawa, M. Ueda, arXiv: 2406.08868.

speaker	Yuki Sakamoto
title	Phase diagrams in evolutionary games on graphs
abstract	Game theory has been applied to various fields such as politics, psychology and biology. In particular, some game-theoretical models are considered to enhance cooperation even in social viscosity in a large population. In recent years, several mechanisms have been found to construct sustainable cooperation by giving players mutual advantages and keeping them from undesirable risk avoidance. One mechanism is a network structure. We consider a multiplayer prisoner's dilemma game on a graphs, where each player is placed at a node and interacts with his/her neighbors connected by edges. Players change their strategis in time based on the pairwise-Fermi update rule. We obtain phase diagrapms with respect to the fraction of cooperators as a function of the strength of natural selection and a possible benefit in the payoff matrix. In the phase diagram, there appears a diamond-shaped mixed region where cooperators and defectors coexist. We discuss how the phase diagram depends on the graph structure and the update rule. [1]: Y. Sakamoto and M. Ueda, Phys. Rev. E 110, 034110 (2024).

speaker	Akihiro Hokkyo
title	Wigner-Araki-Yanase Theorem Beyond Unitary Symmetry
abstract	The Wigner-Araki-Yanase theorem (WAY theorem) asserts that when conserved quantities exist in indirect measurements, it is impossible to accurately measure observables that are not commute with conserved quantities. Conserved quantities are equivalent to continuous unitary symmetries and the theorem can be generalised to the case of discrete unitary symmetries. Therefore, we can say that the WAY theorem represents a restriction that the symmetry of the measurement model imposes on the implemented measurement. However, the WAY theorem in non-unitary symmetries, such as time-reversal symmetries, has not been studied. In this presentation, I discuss a possible extensions of the WAY theorem to non-unitary symmetries. In particular, I show that naive extensions of the WAY theorem do not hold for time-reversal symmetries and that there are certain restrictions due to time-reversal symmetry, when we consider the measurement process rather than just the probability distribution of the measurement.

speaker	Takanao Ishii
title	Tunable Quantum i.i.d. Steady States in Open Quantum Systems
abstract	In closed quantum many-body systems, the quantum phase transition is a dramatic change of the ground state as a function of external parameters. The corresponding phenomenon in open quantum many-body systems is dissipative quantum phase transition, which is the dramatic change of the steady state as a function of external parameters. As the ground state is important in closed quantum many-body systems, calculating the steady state is essential to studying open quantum many-body systems. Time evolution by Hamiltonian creates spatial correlations, while dissipation destroys them. In most of the systems, these two counteracting effects compete with each other, and a finite spatial correlation remains in a steady state. However, in some special cases, the steady state becomes quantum i.i.d. state, which means the absence of spatial correlations. In this seminar, the sufficient condition to have quantum i.i.d. steady state, and the necessary condition to have tunable quantum i.i.d. steady state are discussed. Also, the classification of the system with quantum i.i.d. steady state will be mentioned. Furthermore, calculation of time-correlation functions is to be discussed. The class of system that has quantum i.i.d. steady state would be a great toy model to study nonequilibrium statistical mechanics of quantum many-body systems since the analytical calculations can be easily performed.

speaker	Masaya Nakagawa
title	Derivation of the many-body quantum master equation for ultracold atoms
abstract	Recent development of experimental techniques in atomic, molecular, and optical physics has enabled us to study quantum many-body physics of open systems with unprecedented controllability. A standard tool for the description of Markovian open quantum systems is given by the quantum master equation in the Gorini-Kossakowski-Sudarshan-Lindblad form (a.k.a. the GKSL equation). However, the microscopic derivation of the quantum master equation from the Hamiltonian of a system and an environment is often problematic for many-body systems, because the conventional approximation such as the rotating-wave approximation breaks down. In this seminar, we show that the quantum master equation for many-body systems can be derived under a reasonable assumption for ultracold atoms. On the basis of the microscopic derivation, we clarify the time scales that guarantee the applicability of the quantum master equation to ultracold atoms.

speaker	Koki Shiraishi
title	Derivation of the Davies equation for many-body systems
abstract	The Davies equation [1] is a quantum master equation that describes the relaxation to the thermal equilibrium (Gibbs state). It has been widely used in the fields of thermodynamics and statistical mechanics. It is also applied to quantum algorithms for the Gibbs sampling. Despite the fact that the Davies equation has applications to a wide range of open quantum systems, its derivation breaks down in many-body systems [2]. Therefore, it is a mystery how fundamental properties such as relaxation to the Gibbs state are ensured in an open quantum many-body system. In this talk, we show that the Davies equation can be derived for many-body systems coupled to multiple independent and identical bath. More specifically, we will discuss and present a rigorous derivation in open quadratic systems and numerical results for general chaotic systems. Our results lay the foundation of thermodynamics and equilibrium statistical mechanics in open many-body systems. In the future, the Davies equation will be used to better understand open quantum many-body systems. [1] E. B. Davies Davies, E. Brian. "Markovian master equations." Communications in mathematical Physics 39 91-110 (1974). [2] H. Wichterich, et al. Phys. Rev. E 76, 031115 (2007)

speaker	Atsushi Oyaizu
title	Quantum-classical correspondence between measurement and random spin systems
abstract	Quantum-classical correspondence [1] is one important concept in equilibrium statistical mechanics, which relates quantum systems to classical systems on the level of partition functions. It clarifies fundamental duality between the two, where (imaginary) time direction in the quantum system is identified with one spatial dimension in its classical counterpart. Despite the fundamental importance, quantum-classical correspondence is not always applicable to any classical systems; in particular, within the conventional framework, it is difficult to construct quantum counterparts for classical spin systems with random bond couplings. In this seminar, we demonstrate, with explicit examples, that quantum systems under measurement serve as the counterpart. Given that classical spin systems with random couplings are relevant for the theoretical description of spin glass [2], we also discuss a possible connection to it. [1] M. Suzuki, Prog. Theor. Phys. 46, 1337 (1971); 56, 1454 (1976). [2] K. Binder and A. P. Young, Rev. Mod. Phys. 58, 801 (1986).

speaker	Kazuma Nohara
title	Fisher zero in BCS superconductivity
abstract	The BCS theory, which was proposed by Bardeen, Cooper and Schrieffer, is the fundamental theory that describes superconductivity from a microscopic model. In that theory, a mean-field approximation is used to consider Cooper pairs formed by weak attraction. The self-consistent equation thus obtained is called the gap equation. Despite extensive studies on the respective phenomena in superconductivity, studies on the mathematical origin of the thermal phase transition in superconductivity are scarce. In this study, we investigate Fisher zeros in BCS superconductivity. Fisher‘s zero [1] is one ofthe theoretical approaches to understanding the mechanism of thermal phase transition in terms of zeros of the partition function. We find Fisher zeros drawn in the complex plane of the inverse temperature in a simplified gap equation. We will explain the obtained results and discuss open problems for the future studies. [1] M. E. Fisher, The Nature of Critical Points (University of Colorado Press, Boulder,Colorado, 1965).

speaker	Shozo Yamada
title	Equilibration and revival of general observables in isolated quantum many-body systems
abstract	Isolated many-body systems usually evolve toward equilibrium states. In quantum systems, this process known as equilibration is commonly defined using an infinite-time average. Many works have been conducted to understand equilibration, and it is expected to occur for a wide range of initial states and observables. However, the existing definition of equilibration does not deny special dynamics such as short-time revival. Notably, Ermakov and Fine introduced a concept of almost complete revival (ACR) in a spin-1/2 system [1]. They demonstrated the existence of an initial state where the expectation value of a single-site observable nearly returns to its initial value after a predetermined time. I further extended ACR to general observables in non-integrable quantum systems, and provided a more rigorous argument for its justification. In this talk, I will briefly review the definition of equilibration and some approaches to understand it. Then, I will introduce ACR and its generalization, and present justification of ACR along with numerical simulations. [1] I. Ermakov and B. V. Fine, Phys. Rev. A 104, L050202 (2021).