Academic Year 2024

• Verification of the Haff’s law by granular gas experiments under microgravity environment
  • Hosei O (The University of Tokyo)
    
  • Date:2024/11/11 16:00 --
    
  • Place: YITP room K202
    
  • Abstract:
    粉体ガスと呼ばれる、微小重力環境下における希薄な粉体粒子系は、粒子衝突におけるエネルギー散逸によって特徴的な振る舞いが観察され、非平衡物理学における興味深い対象になっている。我々は、JAXAAsian Try Zero-G 2022 プログラムの下、国際宇宙ステーションにおいて約 4000 個の1.5mm 径銅粒子を100mm径プラスチック球内に封入し、微小重力環境下における自由冷却粉体ガスの時間発展を観察した。画像解析の結果より、緩和の初期過程において、先行研究と同様に銅粒子の平均速度がHaff則に従うことがわかった。主として静電気の影響により、粉体ガスの冷却の終状態で予測されるクラスター形成の観察には至らなかった。本講演では、実験の詳細とともに深層学習を用いた複数粒子追跡の画像解析手法を紹介し、それを用いた解析結果について議論する。 *It is held onsite.
• Information arbitrage in molecular machines
  • David Sivak (Simon Fraser University)
    
  • Date:2024/06/28 15:30 --
    
  • Place:YITP room K202
    
  • Abstract:
    Heat engines and information engines have each historically served as motivating examples for the development of thermodynamics. While these are typically thought of as two separate kinds of machines, recent empirical studies of specific systems have hinted at possible connections. Inspired by molecular machines in the cellular environment, which in many cases have separate components in contact with distinct sources of fluctuations, we study bipartite heat engines and show that they can only produce net output work by acting as information engines. Conversely, information engines can only extract more work than the work consumed to power them if they have access to different sources of fluctuations, i.e., act as heat engines. We illustrate these findings through a close analogy to economic arbitrage, a cyclically controlled 2D ideal gas, the Brownian-gyrator heat engine, and inference of information flows in light-harvesting biomolecular machines. Our results suggest design principles for both heat engines and information engines at the nanoscale, and ultimately imply constraints on how free-energy transduction is carried out within molecular machines.
• Universality and two-body losses: lessons from the effective non-Hermitian dynamics of two particles
  • Alice Marche (Paris-Saclay University)
    
  • Date:2024/05/16 16:00 --
    
  • Place:YITP room K202
    
  • Abstract:
    Two-body losses are one of the most investigated mechanisms that can induce a many-body correlated open quantum dynamics in a bosonic or fermionic degenerate gas. Particular attention in experimental and theoretical analyses has been devoted to the steady properties of the setup and to the late-time dynamics, raising interesting questions about the possible appearance of universal behaviors. In this talk, I will discuss the case of two particles constrained in one spatial dimension and subject to local two-body losses. For this particular few-body system, the dynamics is exactly described by a non-Hermitian Hamiltonian that can be analytically studied, both in the continuum and on a lattice. In these two geometries, we demonstrate the existence of a universal scaling collapse for the time decay of the number of particles, which is valid in the whole parameter space of the problem. In the continuum, the density of particles presents a power-law decay while on the lattice intriguing and unexpected logarithmic corrections appear.
• Some topological aspects of nonequilibrium quantum optical systems【Zoom】
  • Zongping Gong (Tokyo University)
    
  • 2024/05/15 16:00 --
    
  • Online
    
  • Abstract:
    Quantum optical platforms, such as ultracold atoms, trapped ions, and superconducting circuits, provide an ideal playground for exploring the physics of nonequilibrium quantum systems. In this seminar, we will focus on some topological aspects of two fundamental nonequilibrium settings - quench dynamics and non-Hermitian systems, both of which are experimentally accessible in various platforms. In the first part, we will talk about the spacetime topology of quenched topological insulators, especially its signature in the dynamics of entanglement spectrum [1], which is measurable in principle [2]. In the second part, we will discuss the peculiar behaviors of quantum emitters in a lossy bath exhibiting nontrivial non-Hermitian topology [3]. Such kind of systems are realizable by, e.g., superconducting qubits in waveguides [4]. In addition, when there are many quantum emitters forming a regular lattice, the bath can immediate an effective emitter coupling inheriting the topological property [5]. References [1] Z. Gong and M. Ueda, Phys. Rev. Lett. 121, 250601 (2018). [2] Hannes Pichler, Guanyu Zhu, Alireza Seif, Peter Zoller, and Mohammad Hafezi, Phys. Rev. X 6, 041033 (2016). [3] Z. Gong, M. Bello, D. Malz, and F. K. Kunst, Phys. Rev. Lett. 129, 223601 (2022); Phys. Rev. A 106, 053517 (2022). [4] Eunjong Kim, Xueyue Zhang, Vinicius S. Ferreira, Jash Banker, Joseph K. Iverson, Alp Sipahigil, Miguel Bello, Alejandro González-Tudela, Mohammad Mirhosseini, and Oskar Painter, Phys. Rev. X 11, 011015 (2021). [5] Federico Roccati, Miguel Bello, Zongping Gong, Masahito Ueda, Francesco Ciccarello, Aurélia Chenu, and Angelo Carollo, Nat. Commun. 15, 2400 (2024).
• Markovianity and non-Markovianity of Particle Bath with Dirac Dispersion Relation
  • Taira Takano (Tokyo University)
    
  • Date:2024/05/08 16:00 --
    
  • Place:YITP room K206
    
  • Abstract:
    The time evolution of decaying particles to an environment is generally described by a linear integro-differential equation obtained by integrating out the degrees of freedom of the environment from the Schrö dinger equation. When the solution to the equation satisfies the semi-group property, it is known generally as a Markov process[1] and it can be shown to admit exponential decay. Therefore in this talk, I will refer to the deviation from the exponential decay as non-Markovian. The deviation in the short time region become quadratic decay, leading to the quantum Zeno effect [2], and in the long term, oscillatory power law decay, known as long tails [3]. In this lecture, we consider a two-level system with a particle bath that has a Dirac dispersion relation with two key physical parameters, namely the Dirac gap and the spectral upper and lower cut-off, allowing for manipulation of the spectral structure. It is known that non-exponential decay manifest only when the spectrum of the entire Hamiltonian is bounded[4], but the relationship between specific spectral structures and non-Markovian characteristics has been limited, with analysis mainly focused on the pole and branch point structure of the resolvent instead[5,6]. We will show that high energy structure such as spectral cutoff corresponds mainly to the short time quadratic decay whereas the low energy structure such as the Dirac gap strongly correspond to the long time bound state, with no contribution to the behavior of the short time decay. In the gapless case we observe a Markovian decay in infinite cutoff case, confirming the result [3,4]. Surprisingly, we also observe Markovian decay in the finite cut-off case when the time is taken to be a discrete steps. [1] Rivas, A., & Huelga, S. F. (2012). Open quantum systems. [2]Misra, B., & Sudarshan, E. G. (1977). The Zeno’s paradox in quantum theory. Journal of Mathematical Physics, 18(4), 756-763. [3]Khalfin, L. A. (1958). Contribution to the decay theory of a quasi-stationary state. Sov. Phys. JETP, 6(6), 1053-1063. [4]Chiu, C. B., Sudarshan, E. C. G., & Misra, B. (1977). Time evolution of unstable quantum states and a resolution of Zeno's paradox. Physical Review D, 16(2), 520. [5]Garmon, S., Petrosky, T., Simine, L., & Segal, D. (2013). Amplification of non‐Markovian decay due to bound state absorption into continuum. Fortschritte der Physik, 61(2‐3), 261-275 [6]Kofman, A. G., Kurizki, G., & Sherman, B. (1994). Spontaneous and induced atomic decay in photonic band structures. Journal of Modern Optics, 41(2), 353-384.
• Unifying speed limits with optimal transport approach (YITP)
  • Tan Van Vu (Yukawa Institute of Theoretical Physics)
    
  • Date:2024/04/26 16:30 --
    
  • Panasonic Auditorium, Yukawa Hall, Yukawa Institute, Kyoto U. &【Zoom】
    
  • Abstract:
    The study of the speed at which both matter and information propagate is a central focus in quantum mechanics. This subject can be approached through two primary research avenues. The first avenue, pioneered by Mandelstam and Tamm in 1945, led to the concept of quantum speed limits. This concept focuses on the minimum time required for state transformations and has found applications across various scientific domains. The second avenue, initiated by Lieb and Robinson in 1972, introduced the notion of an effective light cone, which pertains to the speed of information propagation within quantum systems. The Lieb-Robinson bound has since proven to be a powerful analytical tool for studying quantum many-body systems. Nevertheless, it is crucial to acknowledge that each of these two concepts has its own set of strengths and limitations. This fact suggests the potential for a novel bound that unifies these concepts, thereby establishing new constraints that neither concept can fully encompass independently. To accomplish this unification, we employ optimal transport theory, enabling us to derive a universal and novel speed limit applicable to a wide range of dynamic scenarios. As a pivotal application of our result, we utilize it to completely resolve a long-standing, open problem concerning bosonic transport in long-range quantum systems.
• Irreversible sampling of glasses Frédéric van Wijland (Université Paris Cité)
  • Frédéric van Wijland (Université Paris Cité)
    
  • Date:2024/04/17 16:00 --
    
  • Place: Conference Room K206, Main Building, Yukawa Institute, Kyoto U. & 【Zoom】＊It will be held in hybrid.
    
  • Abstract:
    When asked to sample a prescribed Boltzmann distribution, the physicist's go-to method of choice is the Metropolis Monte-Carlo algorithm. By using a reversible dynamical evolution (with rules that respect the detailed balance condition), the steady-state is guaranteed to be the target Boltzmann distribution. However, there exist dynamical evolutions that sample the correct Boltzmann distribution while breaking time-reversal. We explore the pros (mostly) and cons (not so many) of resorting to such dynamics to sample dense assemblies of particles. We describe a new algorithm that outperforms state-of-the-art methods for polydisperse systems. ＊It will be held onsite.