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)

---

---

夏学期 / Summer semester (April-July 2025)

<Regular seminars (review of seminal papers)>
    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>
Apr. 10 Oyaizu
    "Infinite Number of Order Parameters for Spin-Glasses"
    G. Parisi, Phys. Rev. Lett. 43, 1754 (1979).
    "The order parameter for spin glasses: a function on the interval 0-1"
    G. Parisi, J. Phys. A: Math. Gen. 13 1101 (1980).
    "Order Parameter for Spin-Glasses"
    G. Parisi, Phys. Rev. Lett. 50, 1946 (1983).

Apr. 17 Sugiura
    "Theory of Superconductivity"
    J. Bardeen, L. N. Cooper, and J. R. Schrieffer, Phys. Rev. 108, 1175 (1957).

Apr. 24 Shiraishi
    "The Theory of a General Quantum System Interacting with a Linear Dissipative System"
    R. P. Feynman and F. L. Vernon Jr. J. Annals of Physics, 24, 118-173 (1963).

May 1 (No seminar)

May 8 (No seminar)

May 15 Hongchao Li
    "Existence of Long-Range Order in One and Two Dimensions"
    P. C. Hohenberg, Phys. Rev. 158, 383 (1967).
    "Absence of Ferromagnetism or Antiferromagnetism in One- or Two-Dimensional Isotropic Heisenberg Models"
    N. D. Mermin and H. Wagner, Phys. Rev. Lett. 17, 1133 (1966).
    "Long-Range Orders in Ground States and Collective Modes in One- and Two-Dimensional Models"
    S. Takada, Prog. Theor. Exp. Phys. 54, 1039-1049 (1975).

May 22 Ishii
    "Dynamical Scaling of Growing Interfaces"
    Mehran Kardar, Giorgio Parisi, and Yi-Cheng Zhang, Phys. Rev. Lett. 56, 9 (1986).

May 29 (No seminar)

Jun. 5 Hokkyo
    "An Area Law for One Dimensional Quantum Systems"
    M. B. Hastings, J. Stat. Mech. 2007, P08024 (2007).
Jun. 12 (No seminar)

Jun. 19 (No seminar)

Jun. 26 Yamada
    "Black holes as mirrors: quantum information in random subsystems"
    P. Hayden and J. Preskill, J. High Energy Phys. 2007, 120 (2007).
Jul. 3 Iyama
    "Theory of dynamic critical phenomena"
    P. C. Hohenberg and B. I. Halperin, Rev. Mod. Phys. 49, 3 (1977).

---

冬学期 / Winter semester (October 2025-January 2026)

   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>
10/2 (No seminar)
10/9 Shiraishi / Oyaizu (@913)
10/16 (No seminar)
10/23 (No seminar)
10/30 Ming Gong
11/6 (No seminar)
11/13 Hongchao Li (from 16:00)
11/20 Hokkyo
11/27 (No seminar)
12/4 (No seminar)
12/11 (No seminar)
12/18 Sugiura
12/25 Nakagawa
1/1 (No seminar)
1/8 Kai Li (@233)
1/15 (No seminar)
1/22 Nakanishi / Yamada (@233)
1/29 Ishii / Iyama (@233)

---

speaker	Koki Shiraishi
title	Quantum master equation approach to electron systems driven by external field or temperature difference
abstract	TBA

speaker	Atsushi Oyaizu
title	Chaotic RG flows in non-unitary quantum dynamics
abstract	One of the most important concepts that underlies the success of renormalization groups (RG) is the universality, where microscopic perturbations do not affect the macroscopic property dramatically. In the language of RG theory, this can be understood by the fact that different systems flows a same fixed point. However, as already pointed out in Wilson's series of papers [1], the convergence of RG flows are not obvious a priori, and a chaotic RG flow without any fixed points is in principle possible. Indeed, it is pointed out, by a simple example of a classical 1D Ising chain under imaginary magnetic field, that if we allow non-Hermiticity in a system, we can easily make the associated RG flows chaotic [2]. However, such a non-Hermiticity is usually difficult to introduce in a real system, and apart from few known examples [3], it has been unclear whether such chaotic RG flows are relevant to a more physical system. Moreover, the physical mechanism of how non-Hermiticity gives rise to chaotic RG flows has remained largely elusive. In this work [4], we first uncover a duality mapping that enables a physical implementation of a non-Hermitian spin chain by nonunitary quantum dynamics. Moreover, by using it, we show that even a single qubit dynamics can accompany such chaotic RG flow in its dynamics. Finally, we reveal the clear physical picture behind the presence of chaotic RG flow in the dynamics. [1] K. G. Wilson, Phys. Rev. B 4, 3174 (1971); Phys. Rev. B 4, 3184 (1971); Wilson & Kogut, Phys. Rep. 12, 2 (1974). [2] Dolan, Phys. Rev. E 52, 4512 (1995). [3] McKay, Berker & Kirkpatrick, Phys. Rev. Lett. 48, 767 (1982), Ilderton, Phys. Rev. Lett. 125, 130402 (2020); Jiang, Qiao & Lan, Phys. Rev. E 103, 062117 (2021). [4] Oyaizu, Li, Nakagawa & Ueda, in preparation.

speaker	Ming Gong
title	A Quantum Circuit Viewpoint on Mesoscopic Partition Noise
abstract	Current noise is a key signal revealing the nature of charge carriers and transport processes. In mesoscopic physics it typically has two contributions: thermal (Johnson–Nyquist) noise and shot noise. In a typical quantum Hall beam splitter, electrons undergo coherent quantum scattering and are partitioned at the outputs, and the partition shot noise can be detected at the receiver even at zero temperature. It is commonly believed that dephasing and relaxation suppress partition noise, an intuition often modeled with the Büttiker-probe approach. However, recent developments show that treating scattering fully quantum mechanically—mapping channels and scattering processes to monitored quantum circuits—can lead to different conclusions; for example, dephasing alone does not suppress shot noise. In this seminar, I will first review the basics of current noise in mesoscopic systems, then introduce recent progress on partition noise from the quantum-circuit perspective. Next, I will outline the conventional understanding when strong energy relaxation is introduced. Finally, I will present our scheme that integrates energy relaxation into the quantum-circuit representation and emphasize our finding that, under strong energy relaxation, shot noise can be fully suppressed, whereas thermal noise cannot; instead, it is partitioned. [1] C. W. J. Beenakker and J.-F. Chen. "Monitored quantum transport: full counting statistics of a quantum Hall interferometer." Quantum 9, 1874 (2025). [2] C. W. J. Beenakker. "Pure dephasing increases partition noise in the quantum Hall effect." arXiv: 2509.10242v1. [3] A. Shimizu and M. Ueda. "Effects of dephasing and dissipation on quantum noise in conductors." Phys. Rev. Lett. 69, 1403 (1992).

speaker	Hongchao Li
title	Optimizing digital quantum simulation of open quantum lattice models
abstract	Simulating quantum many-body systems, as a central topic of high- and low-energy physics, is one of the tasks that quantum computers are naturally suited to solving due to the exponential-to-polynomial speed-ups. Substantial effort has been put into designing quantum algorithms for simulating many-body systems. The state-of-the-art techniques, including the Trotterization methods [1] and quantum signal processing [2], now achieve provably nearly optimal scaling of both geometrically local gate count and parallelized circuit depths with respect to system size and target precision. However, in practice, most physical systems inevitably interact with their surrounding environment, and need to be described as an open quantum system. While near-optimal algorithms have been developed for simulating many-body quantum dynamics, algorithms for their open system counterparts remain less well investigated. In this work [3], we have addressed the problem of simulating geometrically local many-body open quantum systems interacting with a stationary Gaussian environment. Under a smoothness assumption on the system-environment coupling, we develop nearly optimal algorithms for the simulation of non-Markovian dynamics. We additionally show that, if only simulating local observables is of interest, then the circuit depth of the digital algorithm can be chosen to be independent of the system size by introducing the Lieb-Robinson bound of the system. Finally, for the Markovian dynamics with commuting jump operators, we propose two algorithms based on a locally dilated Hamiltonian construction and on sampling a Wiener process, respectively. These algorithms reduce the asymptotic gate complexity on the system size compared to currently available algorithms in terms of the required number of geometrically local gates. [1]: A. M. Childs and Y. Su, Phys. Rev. Lett. 123, 050503 (2019). [2]: J. Haah, M. B. Hastings, R. Kothari, and G. H. Low, SIAM Journal on Computing 35, FOCS18 (2021). [3]: X. Yu, H. Li, J. I. Cirac and R. Trivedi, arXiv: 2509.02268.

speaker	Akihiro Hokkyo
title	Rigorous Test for Quantum Integrability and Nonintegrability
abstract	The integrability of a quantum many-body system, characterized by the presence or absence of local conserved quantities, drastically affects the dynamics of isolated systems, including thermalization. Nevertheless, a rigorous and comprehensive method for determining integrability or nonintegrability has remained elusive. In this talk, we address this challenge by introducing rigorously provable tests for the integrability and nonintegrability of quantum spin chains with finite-range interactions. First, in the first half of the talk, based on [1], we show that the absence of solutions to a certain local equation implies nonintegrability. This leads to a system-size-independent algorithm that guarantees nonintegrability for translationally invariant systems. Next, in the second half of the talk, based on [2], we show that a single conserved quantity known as the Reshetikhin condition provides a sufficient condition for integrability. This, in particular, shows that the entire hierarchy of conservation laws associated with solutions of the Yang-Baxter equation is already encoded in the lowest nontrivial conservation law. Combining these two results, I will also discuss the sharp boundary between integrable and nonintegrable quantum spin chains. [1]: A. Hokkyo, Rigorous Test for Quantum Integrability and Nonintegrability, arXiv:2501.18400. [2]: A. Hokkyo, Integrability from a Single Conservation Law in Quantum Spin Chains, arXiv: 2508.20713.

speaker	Shuma Sugiura
title	Feedback Cooling of a Levitated Nanoparticle
abstract	Cooling a nanoparticle—namely, suppressing fluctuations in the motion of its center of mass—is expected to enable further exploration of quantum phenomena in nanoparticles and to serve as the basis for highly sensitive accelerometers, gyroscopes, and sensors for electric fields, gravitational waves, and related signals. In this seminar, I introduce several feedback schemes for cooling a nanoparticle. The first scheme is a revised linear–quadratic–Gaussian (LQG) control, inspired by the low-pass-filter feedback proposed in our previous paper [1]. The key idea is to incorporate the feedback potential directly into the cost function. I show that the optimal control derived from this revised cost function can cool a nanoparticle to an energy lower than that achievable with conventional schemes. Although this revised LQG control provides the optimal solution, its practical implementation is challenging because implementing the Kalman filter on FPGA hardware requires a complex and nontrivial design. To address this issue, I propose two alternative feedback schemes that are readily compatible with experiments. One is feedback cooling using a first-order low-pass filter, and the other uses a fourth-order low-pass filter. I also compare these feedback schemes in terms of achievable phonon occupancy, the number of parameters requiring fine-tuning, and related performance metrics. [1] S. Sugiura and M. Ueda. arXiv preprint arXiv:2505.10157 (2025).

speaker	Masaya Nakagawa
title	Topology of feedback control in classical stochastic processes
abstract	Topology provides a versatile tool for characterizing phases of matter in a wide variety of quantum and classical systems, both in and out of equilibrium. In this seminar, we introduce a topological characterization of classical stochastic processes described by discrete-time Markov chains [1]. In particular, we uncover a topological aspect of a Maxwell-demon experiment in Ref. [2] by demonstrating that the unidirectional transport induced by feedback control in this experiment has a topological origin. We further show that appropriately designed feedback protocols can achieve topologically nontrivial stochastic processes that cannot be realized by any continuous-time Markov process due to a topological obstruction. Finally, we discuss fundamental differences between classical and quantum feedback control [3] from a topological perspective. [1] M. Nakagawa and M. Ueda, in preparation. [2] S. Toyabe et al., Nat. Phys. 6, 988 (2010). [3] M. Nakagawa and M. Ueda, Phys. Rev. X 15, 021016 (2025).

speaker	Kai Li
title	Quantum elastic gas
abstract	Since the realization of Bose–Einstein condensation, ultracold bosonic gases have largely been explored in two interaction limits: short-range isotropic contact interactions and long-range anisotropic dipolar interactions. We propose a new class of bosonic gas with short-range anisotropic interactions motivated by anisotropic van der Waals forces in certain atomic species (e.g., main group III atoms), and derive the corresponding low-energy anisotropic pseudopotential. Using Bogoliubov and mean-field theory with an ultraviolet cutoff, we obtain a phase diagram featuring a stable isotropic condensate and an unstable density-wave regime. Time-dependent GP equation simulations show that, in the unstable regime, the system exhibits lower-dimensional local collapse along the axial or transverse directions rather than a global three-dimensional collapse.

speaker	Ken Nakanishi
title	Attension Mechanisms in Large Language Models
abstract	TBA

speaker	Shozo Yamada
title	Tunable many-body burst in isolated quantum systems
abstract	Thermalization processes in isolated quantum many-body quantum systems depend on initial states and are not necessarily monotonic. In this work [1], we propose two numerical methods to construct a low-entangled initial state that creates a "burst"---a temporal deviation of a local observable from its thermal equilibrium value---at a targeted time for a given Hamiltonian. We apply these methods to show that a burst can be observed at short times for a nonintegrable local Hamiltonian. The burst is accompanied by persistent local information as indicated by a negative entanglement growth. We also analytically show that a burst becomes probabilistically rare at long times. Our results suggest that a nonequilibrium state is maintained for a particular initial state until being overtaken by quantum scrambling, a prediction testable in quantum simulators. [1] S. Yamada, H. Hokkyo, M. Ueda, in preparation.

speaker	Takanao Ishii
title	NISQ simulation of open quantum many-body systems
abstract	Quantum computation is drawing attention in computational physics, as efficient quantum algorithms may outperform classical methods, enabling simulations of larger system size with higher accuracy. In this work, we explore the use of NISQ devices for simulating open quantum many-body systems, a direction expected to yield new insights previously unattainable with classical computational approaches. We adopt a method based on the Stinespring representation to implement non-unitary dynamics. Although this method scales better with system size than other methods, its application on NISQ devices has been limited due to the heavy reliance on qubit reset operations. Recent improvements in reset errors of some devices, however, have made this approach more feasible. We further examine error mitigation techniques for dynamical circuits and discuss the applicability of NISQ devices to the study of dissipative quantum phase transitions based on experiments performed on an IBM quantum device.

speaker	Naoki Iyama
title	Analysis of Grokking: From empirical Observation to Theoretical Insight
abstract