Academic Year 2025

• Thermodynamics of counting observables in stochastic excursions
  • Guilherme Fiusa (University of Rochester)
    
  • 2025/12/17 16:00 --
    
  • Seminar Room K202, Main Building, Yukawa Institute, Kyoto U.
    
  • Understanding fluctuations of observables across stochastic trajectories is essential for various fields of research, from quantum thermal machines to biological motors. We introduce a framework to analyze the statistics of counting observables in sub-trajectories—dubbed as stochastic excursions—of processes out of equilibrium. Given a partition of the state space into two sets A and B, an excursion is defined as the segment of the trajectory that starts with a transition from A to B and ends upon the first return from B to A. Our approach offers analytical expressions for the full distribution of counting observables (such as currents, heat, work, entropy production, and dynamical activity) and the excursion duration, capturing their correlations and finite-time fluctuations. As our main result, we uncover a nontrivial fundamental relation between fluctuations of counting observables at the single-excursion level and the steady state noise obtained from full counting statistics, offering a tool to inspect noise sources. We also show the existence of a fluctuation theorem and a thermodynamic uncertainty relation at the level of individual excursions. We discuss examples from distinct fields in which the excursion framework naturally addresses relevant questions, and explore in more detail how analyzing excursions yields additional insights into the operation of the three-qubit absorption refrigerator and transport in double quantum dots.
    
• 集中講義：基礎物理学研究所物理学第一分野特別講義
  • 森 貴司 (慶應義塾大学理工学部物理学科)
    
  • 2025/11/04 --- 2025/11/06
    
  • Conference Room K206, Main Building, Yukawa Institute, Kyoto U.
    
  • 本講義では、開放量子系の理論的な枠組みを詳しく解説する。特に、量子マスター方程式の微視的導出およびその基本的性質について学ぶ。近年、量子多体系における散逸の影響に大きな関心が持たれていることを踏まえて、本講義の後半では着目系自体が量子多体系とする設定を扱う。
    授業計画と内容
    1. 開放量子系の一般論
    2. 古典・量子Brown 運動
    3. 小さな量子系におけるLindblad 方程式
    4. 量子多体系におけるLindblad 方程式
    5. 開放量子多体系の基本的性質
    6. Lindblad 方程式の厳密な導出に向けて
    時間割：
    11月 4日(火) 10:30～12:00　13:15～14:45　15:00～16:30
    11月 5日(水) 10:30～12:00　13:15～14:45　15:00～16:30　16:45～18:15　セミナー
    11月 6日(木) 10:30～12:00　13:15～14:45　15:00～16:30
    
    
• Remarkable aspects about stochastic thermodynamics: Collective systems, Pareto-fronts and finite thermal reservoirs
  • Carlos E. Fiore (University of São Paulo)
    
  • 2025/10/29 16:00 --
    
  • Seminar Room K202, Main Building, Yukawa Institute, Kyoto U.
    
  • The study of energy conversion at microscopic scales is central in nonequilibrium thermodynamics, such as physics, chemistry, biology, and quantum technologies. In this talk, I’ll discuss topics in stochastic thermodynamics of collective systems. Unlike typical phase transitions characterized by a single critical point, we show that the interplay between two temperatures and biased forces can split the order-disorder transition into two different points, depending on the dominant ordered state. We also show that, under general conditions, the heat engine is delimited by the minimal power fluctuations and entropy production, which together delimit its optimal performance—achieved when these conditions are fully satisfied. Finally, we explore a multi-objective optimization framework using Pareto fronts, revealing that while few-variable optimization leads to convex fronts, increasing complexity and nonconservative forces result in concave fronts.
    
• Optimally Fast Qubit Reset
  • Yue Liu (YITP, Kyoto University)
    
  • 2025/10/03 16:00 --
    
  • Seminar Room K202, Main Building, Yukawa Institute, Kyoto U.
    
  • In practice, qubit reset must be operated in an extremely short time, which incurs a thermodynamic cost within multiple orders of magnitude above the Landauer bound. We present a general framework to determine the minimal thermodynamic cost and the corresponding optimal protocol for memory erasure under arbitrary erasure speeds. Our study reveals the divergent behavior of minimal entropy production in the short-time limit depends on the convergence and divergence of the jump operator. There is an inherent trade-off between the minimal required time and the set error probability for the convergent class. Moreover, we find the optimal protocol exhibits general features in the fast-driving regime. To illustrate these findings, we employ fermionic and bosonic baths as examples. Our results suggest that the superOhmic bosonic heat bath is suitable for qubit reset.
    
• Exact Results in Stochastic Processes with Division, Death, and Diffusion
  • Elsen Tjhung (The Open University)
    
  • 2025/08/07 16:00 --
    
  • Seminar Room K202, Main Building, Yukawa Institute, Kyoto U.
    
  • We consider a generic class of stochastic particle-based models whose state at an instant in time is described by a set of continuous degrees of freedom (e.g. positions), and the length of this set changes stochastically in time due to birth-death processes. Using a master equation formalism, we write down the dynamics of the corresponding (infinite) set of probability distributions: this takes the form of coupled Fokker-Planck equations with model-dependent source and sink terms. We derive the general expression of entropy production rate for this class of models in terms of path irreversibility. To demonstrate the practical use of this framework, we analyze a biologically motivated model incorporating division, death, and diffusion, where spatial correlations arise through the division process. By systematically integrating out excess degrees of freedom, we obtain the marginal probability distribution, enabling exact calculations of key statistical properties such as average density and correlation functions. We validate our analytical results through numerical Brownian dynamics simulations, finding excellent agreement between theory and simulation. Our method thus provides a powerful tool for tackling previously unsolved problems in stochastic birth-death dynamics.
    
  • References
    Reference: S. Cameron and E. Tjhung, arXiv:2503.13150
• Continuum modeling of large deformation granular materials
  • Bodhinanda (Nanda) Chandra (University of California at Berkley)
    
  • 2025/07/30 16:00 --
    
  • Conference Room K206, Main Building, Yukawa Institute, Kyoto U. & 【Zoom】
    
  • This seminar will discuss continuum modeling approaches for large-deformation problems in granular flows. In particular, it will present recent developments in the Material Point Method (MPM), a mesh-particle hybrid approach, to simulate granular flow dynamics, with a focus on geomaterial behavior and geohazard scenarios. These simulation techniques are vital for enhancing infrastructure resilience in the face of climate change, especially in near-water environments. The presentation will highlight key modeling challenges and explore future directions for addressing other complex problems involving granular materials under extreme conditions.
    
• From flowing to static: statistics of elasto-plastic transitions
  • Stefan Luding (Univ. of Twente)
    
  • 2025/07/09 15:15 --
    
  • Seminar Room K202, Main Building, Yukawa Institute, Kyoto U. & 【Zoom】
    
  • How do soft granular materials (or dense amorphous systems) respond to externally applied deformations at different rates – from fast to slow to very slow – and for different system sizes? This long-standing question was intensively studied for shear deformation modes, but only more recently also for isotropic deformations, like compression-decompression cycles [1,2]. For moderate strain rates, in the solid-like state, above jamming [3,4,5], the system appears to evolve more or less smoothly in time/strain, whereas for slow enough deformations, the material flips intermittently between the elastic, reversible base-state and plastic, dynamic “events”. Only during the latter events the micro-structure changes, it re-arranges, irreversibly. The reversible base state involves both affine and non-affine deformations, while the events are purely non-affine. Besides their phenomenology and statistical properties, in particular, the system size and rate dependence [6] of the events is studied, providing reference data, to be compared in future to experiments on model materials like hydrogel particles using modern techniques. Finally, perspectives and relations to real materials in application are to be addressed. Figure 1 displays the affine, non-affine, and total displacement fields, where in the center of the event (much larger localized displacements) the particles are highlighted. Figure 2 displays the kinetic to potential energy ratio during compression from below jamming to above, for various different system sizes and strain-rates. The zoom-in in Fig. 2 (right) allows to observe isolated events (for slow enough compression rate) and their exponential decay of granular temperature (dynamic cooling) relaxing towards the steady, smooth, elastic situation between events. The larger the system size, the more events occur, overlapping in time (strain) if the compression rate is too fast.
    
  • References
    [1] K. Taghizadeh, S. Luding, R. Basak, L. Kondic, Understanding slow compression of frictional granular particles under slow compression by network analysis, Soft Matter (submitted 2023)
    [2] S. Luding, K. Taghizadeh, C. Cheng, L. Kondic, Understanding slow compression and decompression of frictionless soft granular matter by network analysis, Soft Matter 18, 1868 (2022)
    [3] S. Luding, Granular matter: so much for the jamming point, Nature Physics 12, 531-532, 2016
    [4] N. Kumar, S. Luding, Memory of jamming -- multiscale models for soft and granular matter, Granular Matter 18, 58, 2016
    [5] S. Luding, Y. Jiang, and M. Liu, Un-jamming due to energetic instability: statics to dynamics, Granular Matter 23, 80, 2021
    [6] S. Luding, How does static granular matter re-arrange for different isotropic strain rate?, in Powders & Grains 2021 – EPJ Web of Conferences (2021), Vol. 249, p. 10001
    [7] S. Luding, Elastic-plastic intermittent re-arrangements of frictionless, soft granular matter under very slow isotropic deformations, Frontiers Physics 11, 1211394, 2023
• Mini-workshop on "Jamming, rheology and granular matter"
  • 2025/07/09 --- 2025/07/09
    
  • YITP, Room K202
    
  • A collection of macro-sized particles resembling sand, which does not relax to thermal equilibrium, is known as a athermal particle system and is recognized for its unique behaviors. Additionally, as can be seen from the fact that the Earth's surface is covered by particulate matter, controlling such systems is extremely important. They have extensive applications in fields such as pharmacy, chemical engineering, mechanical engineering, and Earth sciences, and are also attracting attention as subjects of fundamental physics. This research meeting will be held on the occasion of Stefan Luding's visit, the managing editor of the specialized journal Granular Matter and president of the global community AEMMG, to conduct a one-day mini-workshop and facilitate information exchange.
    
• YITP Colloquium: Kardar-Parisi-Zhang equation: one and two components
  • Herbert Spohn (Technical University of Munich)
    
  • 2025/06/12 15:30 --
    
  • Panasonic Auditorium, Yukawa Hall, Yukawa Institute, Kyoto U. ＆ Online.
    
  • The KPZ equation was first written down in 1985 and has a rich history, reaching physical applications beyond the originally envisioned surface growth. In my lecture, I will discuss recent advances. Amongst others, they include the extension to two components and the riddle of the space-time spin-spin correlations of the Heisenberg spin chain.
    
• International Molecule-type Workshop "Hydrodynamics of low-dimensional interacting systems: Advances, challenges, and future directions"
  • 2025/06/02 --- 2025/06/13
    
  • Panasonic Auditorium, Yukawa Hall, YITP
    
  • The two complementary views of the dynamics of a many-body system are the microscopic perspective, based on Newton’s or Schrödinger’s equations, and the macroscopic perspective, based on hydrodynamic equations such as the Euler or Navier-Stokes equations. There has recently been significant interest in formulating hydrodynamics to understand physical phenomena in low dimensions and so-called integrable systems. Two important new fields have emerged: (i) nonlinear fluctuating hydrodynamics and (ii) generalized hydrodynamics. One of the most intriguing observations in both cases is anomalous transport and its connections with the Kardar-Parisi-Zhang equation. The workshop will explore some of the outstanding open questions in these two fields. This workshop will take place during Prof. Abhishek Dhar’s stay at YITP as a visiting professor. We will also invite Prof. Herbert Spohn, a well-known mathematical statistical physicist.
    
• Half century of theory of synchronization
  • Yoshiki Kuramoto (Emeritus professor, Kyoto University)
    
  • 2025/05/29 16:00 --
    
  • Panasonic Auditorium, Yukawa Hall, Yukawa Institute, Kyoto U. & Online
    
  • 私の研究史は振動子集団の同期理論を中心に展開してきましたが、その原点となるのは、Brusselatorと呼ばれる反応拡散モデルから複素Ginzburg-Landau方程式を中心多様体縮約法によって導出した短い論文にあります (Y.Kuramoto & amp; T.Tsuzuki, 1974)。以来、比較的よく言及される私の貢献（特に３つを採り上げます）のほとんどは、この方程式またはその変形判に位相縮約法を適用することによって得られた理論モデルが基になっています。この二重の縮約によって得られたきわめてシンプルな振動子モデルが、今日に至るまでの同分野および関連分野の発展にどのように影響し得たかを論じます。その具体的過程をお話しすることで、非線形発展方程式の縮約という概念が複雑な現象世界を探求する上で果たす役割の重要さを理解していただければと思います。
    
• YITP Colloquium：Page curve entanglement dynamics of a gas of freely expanding non-interacting fermions
  • Abhishek Dhar (International Centre for Theoretical Sciences / YITP, Kyoto University)
    
  • 2025/05/22 16:00 --
    
  • Conference Room K206, Main Building, Yukawa Institute, Kyoto U.
    
  • We consider a gas of non-interacting fermions that is released from a box into the vacuum and look at the entanglement between the escaped particles with those in the box. This provides a simple analytically tractable model that reproduces many features of the Page curve characterizing the evolution of entanglement entropy during evaporation of a black hole. Apart from the entropy we consider several other physical observables and show that the framework of generalized hydrodynamics provides a rather surprisingly accurate description of the quantum dynamics. We also report numerical results for interacting spin chains.
    
• Hydrodynamics of eigenvalues of large random matrices
  • Frédéric van Wijland (Université Paris Cité)
    
  • 2025/05/21 16:30 --
    
  • Conference Room K202, Main Building, Yukawa Institute, Kyoto U.
    
  • Consider a large matrix with independent random entries. In 1955 Wigner showed that, in a suitably rescaled way, the histogram of eigenvalues eventually converges to the celebrated semi-circle law. Shortly after, in 1962, Dyson endowed the matrix elements with a Langevin dynamics of their own thereby turning the static exploration of the properties of the eigenvalues into a dynamical one. With a focus on collective modes, we will begin by recalling the basic landmarks of random matrix theory. Then we shall describe how to construct a noisy hydrodynamic approach to describe the fluid of eigenvalues. Quite remarkably, we will show how to get an exact expression for the dynamical structure factor of that fluid that extends to the time domain the equilibrium result of Brézin and Zee of 1993. Work done in collaboration with G. Téllez-Acosta and K. Mallick.
    
• Orientational ordering in a two-dimensional active model for dense bacterial suspension
  • Yoshihiko Nishikawa (Kitasato University)
    
  • 2025/05/21 15:00 --
    
  • Conference Room K202, Main Building, Yukawa Institute, Kyoto U.
    
  • Active matter systems often exhibit dynamic and static properties that are highly distinct from thermal equilibrium, including motility-induced phase separation, active turbulence, and crystallization in low dimensions. Bacterial suspension is one typical example, in which self-propelled bacteria move and interact with others, leading to complex, emergent behaviors. Recently, H. Lama et al. (2024) experimentally showed that two-dimensional dense suspension of E. Coli has two glassy transitions at different densities, where the orientational and translational degrees of freedom become dynamically arrested, respectively. They further found that the exponent for the critical divergence of the relaxation time is smaller than the lower-bound of the mode-coupling theory for equilibrium glassy systems. While this suggests the glassy transition to be qualitatively different from the equilibrium counterpart, the origin of the small exponent remains unclear. Here, we propose a minimal active model for dense bacterial suspension in two dimensions, and numerically study its dynamics and statics. In our model, each bacterium is represented by a spherocylinder of fixed length, with its state specified by position of the center of mass and orientation. Bacteria interact with each other via a short-range repulsive interaction and actively move in the direction of its orientation. To mimic the tumbling motion of bacteria in a crowded environment, we further incorporate in the model stochastic velocity reversal with a fixed rate per unit time. With increasing density, the orientational dynamics of the system drastically slows down and its relaxation time shows a rapid growth, suggesting the critical divergence at a finite density \phi_c. On the other hand, even at \phi_c, the translational dynamics has a short relaxation time and bacteria can easily change their positions, with their orientations virtually fixed for a very long time. These dynamical properties are consistent with the experiment. Despite the long time scale for the bacterial orientation, the nematic order remains short ranged, again consistent with the experimental results. However, we find that the length scale of the `tetratic', fourfold orientational order grows much faster than that of the nematic order when approaching \phi_c, indicating that the slow orientational dynamics is controlled by this orientational order. We will also discuss the collective orientational and translational dynamics near \phi_c.
    
• Mean-field Theory Becomes Exact Under Shear Flow: A Dynamic Renormalization Group Study of the O(n) Model
  • Harukuni Ikeda (YITP, Kyoto University)
    
  • 2025/05/07 16:00 --
    
  • Conference Room K202, Main Building, Yukawa Institute, Kyoto U.
    
  • Mean-field theory, such as Landau theory, provides a simple yet powerful framework for understanding critical phenomena. However, in equilibrium systems, its predictions often fail in low dimensions: the upper critical dimension is typically four, and significant deviations in critical exponents are observed in two and three dimensions. Intriguingly, experiments on phase separation under shear flow and recent simulations of spin models have reported mean-field-like critical exponents even in two dimensions [1, 2]. These findings suggest that shear flow may fundamentally alter the nature of critical behavior, effectively lowering the upper critical dimension. To uncover the physical mechanism behind this phenomenon, we performed a dynamic renormalization group analysis of the O(n) model under simple shear flow [3]. A key innovation of our work is the explicit treatment of shear-induced anisotropy in the scaling analysis—an aspect neglected in earlier studies [4]. Our results reveal the existence of a novel Gaussian fixed point governed by strong anisotropy due to shear. Strikingly, we find that the upper critical dimension of this fixed point is reduced to two or lower for both conserved and non-conserved dynamics. This implies that mean-field theory becomes asymptotically exact in all dimensions except one.
    
  • References
    [1] D. Beysens, M. Gbadamassi, and L. Boyer Light-Scattering of a Critical Mixture with Shear Flow, Phys.Rev.Lett. 43 (1979) 1253
    [2] H. Nakano, Y. Minami, and S. Sasa Long-Range Phase Order in Two Dimensions under Shear Flow, Phys.Rev.Lett. 126 (2021) 1606 04
    [3] H. Ikeda, and H. Nakano, Dynamical renormalization group analysis of O(n) model in steady shear flow, arXiv:2412.02111 (2024)
    [4] A. Onuki, and K. Kawasaki, Nonequilibrium Steady State of Critical Fluids under Shear Flow: A Renormalization Group Approach, Ann. Phys. 121 (1979) 456
    
• Unidirectional Transport Effect in non-Hermitian Non-reciprocal System: A General NEGF Approach
  • Sumit Kumar Jana (Indian Institute of Technology Hyderabad)
    
  • 2025/04/21 16:00 --
    
  • Conference Room K202, Main Building, Yukawa Institute, Kyoto U.
    
  • The emergence of dissipation and noises in quantum systems poses a major technological challenge. As an alternative, we focus on designing quantum systems that utilize dissipation and noise advantageously for the same purpose. We developed a non-Hermitian Hamiltonian with frequency-dependent non-reciprocal hopping analogous to the Hatano-Nelson model, by integrating out bath degrees of freedom in the microscopical lattice bath model. Our framework presents a novel feature of frequency-dependent unidirectional transport phenomena beyond the Markovian regime, also we uncover the emergence of skin effect at a specific frequency. We study the transport phenomena by employing the non-equilibrium Green’s function technique. These findings provide a new perspective on leveraging dissipation to achieve controlled directional transport in quantum systems.
    
• Constructing an interface equation, in or out of equilibrium
  • Lila Sarfati (Université Paris Cité)
    
  • 2025/04/16 16:00 --
    
  • Conference Room K202, Main Building, Yukawa Institute, Kyoto U.
    
  • Abstract: Given a mesoscopic description where a field admits two stationary solutions, we are interested in the interface separating the two states. In most cases, the dynamics of this interface is described by the Edwards-Wilkinson (EW) equation. However in the presence of additional symmetries or conservation laws, the description departs from the EW one, as already understood in the 1980's (Langer and Turski, Kawasaki and Ohta). Because obtaining the interface dynamics is technically challenging, several authors (Bray et al, Fausti et al) have introduced a mathematical shortcut yielding results consistent with the correct behavior in several standard models. In this presentation, I will review the existing literature and I will show not only that such shortcuts have unphysical features, but also that a fully systematic method, built on procedures developed by Kawasaki and Ohta and Bausch et al, can be implemented. This method is applied in and out of equilibrium, both to linear order and beyond.
    
• Measurement Induced Gain and Lasing
  • Apan Dinda (Indian Institute of Technology Hyderabad)
    
  • 2025/04/09 16:00 --
    
  • Conference Room K206, Main Building, Yukawa Institute, Kyoto U.
    
  • Abstract: We discuss a framework based on the measurement and feedback protocol where the feedback energy is stored inside the cavity interacting with a single qubit via Jaynes-Cummings coupling. We include cavity loss as a repeated interaction model with an imaginary qubit. This can be thought to model a detector. In this setting, we find that the gain from the feedback can take the cavity to the lasing regime, with Poissonian photon distribution. This nonlinear behavior is well captured within our repeated interaction scheme. On taking the continuous limit, one obtains a Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) master equation, which, however, is unable to explain long time Poissonian photon distribution. We further analyze the nature of the light in the cavity using Glauber's coherence function and Wigner Quasi-probability distribution.