2022年度セミナー Seminars in 2022
久しぶりに対面での研究室セミナーを復活しました。 今年度後期は水曜日午後に生産技術研究所（柏キャンパス）研究実験棟Ⅰの３階大会議室で行います。 このセミナーの他にも、毎週金曜日１０時半からオンラインで行われる統計力学セミナーにも参加しています。
We have revived our on-site seminar series. The second half of this year, we do regular seminars from 2:30pm, every Wednesday at the large conference room on the third floor of Research and Testing Complex I, Institute of Industrial Science (Building 30 of this map). We also attend the online seminar series Statistical Physics seminar series from 10:30am every Friday.
| 日程 Date |
時間 Time |
講演者 Speaker |
演題・Abstract Title and Abstract |
| 10月19日(水) Wed, 0ct 19 |
14:00 |
平良敬乃さん（東大） Dr. Takano Taira (U. Tokyo) |
The breakdown of the Higgs mechanism at the exceptional point Abstract |
| 10月26日(水) Wed, Oct 26 |
14:15 |
多賀圭理さん（早大） Mr. Keisuke Taga (Waseda U.) |
Koopman operator analysis for the elementary cellular automata Abstract |
| 11月2日(水) Wed, Nov. 2 |
13:00 |
Dr. Thierry Martin (U. Aix-Marseille) |
Quantum transport in conventional and topological superconducting hybrid systems Abstract |
| 11月9日(水) Wed, Nov. 9 |
14:30 |
望月健さん（理研） Dr. Ken Mochizuki (RIKEN) |
Complexity transitions in non-unitary boson sampling dynamics Abstract |
| 11月16日(水) Wed, Nov. 16 |
13:00 |
羽田野直道 Naomichi Hatano |
Arrow of time in quantum mechanics Abstract |
| 12月14日(水) Wed, Dec. 14 |
14:30 |
曽根和樹さん（東大） Mr. Kazuki Sone (U. Tokyo) |
To be announced. Abstract |
第０１回 No. 01
講師 Speaker: 平良敬乃さん（東京大）Dr. Takano Taira (IIS, The University of Tokyo)日時 Date: １０月１９日（水）午後２時〜 Wed, Oct 19, 2:00pm
演題 Title: The breakdown of the Higgs mechanism at the exceptional point
要旨 Abstract: The non-Hermitian physics has seen intensive theoretical and experimental development over the past decades. One of the key features of the non-Hermitian system is the branch point of the eigenvalue called the exceptional point, where the rank of the non-Hermitian operator is reduced. We will briefly review PT-symmetric quantum mechanics and present a non-Hermitian extension of the Yang-Mills theory coupled with Higgs fields. We will show that the classical mass of massive gauge and t'Hooft-Polyakov monopole vanishes at the exceptional point while the vacuum solution stays finite.
第０２回 No. 02
講師 Speaker: 多賀圭理さん（早大） Mr. Keisuke Taga (Waseda U.)日時 Date: １０月２６日（水）午後２時１５分〜 Wed, Oct. 26, 2:15pm
演題 Title: Koopman operator analysis for the elementary cellular automata
要旨 Abstract: The Koopman operator theory for analyzing nonlinear dynamical systems has attracted much attention recently. The Koopman operator is a linear operator which gives the time evolution of the observables of the dynamical system, thus, we can analyze nonlinear systems by using the methods for linear systems. On the other hand, because the Koopman operator acts on a function space of observables, it can be generally infinite-dimensional and difficult to analyze. In this talk, I will introduce the Koopman operator for the finite state system. In this case, the Koopman operator is finite-dimensional, and we can derive a finite-dimensional representation of the Koopman operator and construct the Koopman eigenvalues and Koopman eigenfunctions explicitly. I will show that the Koopman analysis of finite state systems can reveal fundamental properties of the system, such as reversibility and conserved quantities. I will also introduce dynamic mode decomposition (DMD), a data-driven method to obtain an approximation of the Koopman operator from time-series data, for the finite state system. The theoretical results will be applied to elementary cellular automata, which have been studied as models of real-world systems, such as pigmentation patterns on shells, peeling patterns of adhesive tapes, and congestion dynamics of traffic flow as examples of the simplest system exhibiting spatiotemporal dynamics.
第０３回 No. 03
講師 Speaker: Dr. Thierry Martin (U. Aix-Marseille)日時 Date: １１月２日（水）午後１時〜 Wed, Nov. 2, 1:00pm
演題 Title: Quantum transport in conventional and topological superconducting hybrid systems
要旨 Abstract: In nano-physics, an electron can be transferred into the gap of a superconductor provided that it is accompanied by another electron in order to form Cooper pairs. I will review a few phenomena where this Andreev reflection processes play a role and examine how transport through topological superconductors can be illustrated to test the relevance of Majorana fermions.
第０４回 No. 04
講師 Speaker: 望月健さん（理研）Dr. Ken Mochizuki (RIKEN)日時 Date: １１月９日（水）午後２時３０分〜 Wed, Nov. 9, 2:30pm
演題 Title: Complexity transitions in non-unitary boson sampling dynamics
要旨 Abstract: Non-Hermitian quantum mechanics have been extensively studied as an effective theory for open quantum systems exhibiting non-unitary dynamics, and various intriguing phenomena have been revealed. Non-Hermitian dynamics has been observed not only in genuinely quantum systems but also in classical systems. In that sense, it is a fundamental but largely unexplored question to what extent non-He rmitian quantum mechanics exhibits unique quantum nature distinct from classic al systems. In isolated quantum systems, unique quantum nature can be discusse d by its computational complexity. It is investigated in the boson sampling problem, where probability distributions of bosons can be hard to sample by classical computers.
In the seminar, I report novel transitions concerning the computational complexity in non-unitary dynamics of bosons. We find that PT-symmetry breaking, which is a unique phenomenon in non-Hermitian open quantum systems, is profoundly related to the computational complexity of the boson sampling problem. In the PT -broken phase, there is a dynamical transition where the probability distribution of bosons becomes approximated by that of distinguishable particles, which leads to the easiness of the boson sampling with classical computers. Thus, PT symmetry breaking makes bosonic systems enter classical regime. We discuss that the enhancement of the classical nature in terms of the easiness for sampling is ensure d in the long-time regime in a wide range of non-unitary boson sampling dynamics.
参考文献 References
Ken Mochizuki and Ryusuke Hamazaki, arXiv:2207.12624
第０５回 No. 05
講師 Speaker: 羽田野直道 Naomichi Hatano日時 Date: １１月１６日（水）午後１時〜 Wed, Nov. 16, 1:00pm
演題 Title: Arrow of time in quantum mechanics
要旨 Abstract: Why does the time flow only in one particular direction? This is a very big and fundamental question of physics, called the problem of "the arrow of time." There are a few types of the arrow of time. The cosmological arrow of time refers to the direction of time in our universe as we see it. The psychological arrow of time refers to the direction of time that we perceive.
The arrow of time that I am going to discuss in the present talk is the dynamical arrow of time, which refers to the following problem. The fundamental equations of motion, namely the Newton equation for classical mechanics and the Schroedinger equation for quantum mechanics, are symmetric if we flip the arrow of time. Nonetheless, we see the oscillation of a pendulum always diminishes and an excited state always decays into the ground state. How can this inconsistency arise? There have been many answers to it but most of them attribute the reason to the limitation of our ability of measuring physical systems. My collaborator Gonzalo Ordonez and I have reservations because it almost sounds like the dynamical arrow of time could be flipped or eliminated if we have more abilities!
We answer the question of the dynamical arrow of time in the realm of quantum mechanics in the following way. We succeeded in deriving a time-reversal symmetric decomposition of the time evolution operator which contains both decaying states and growing states symmetrically. If we watch the time evolution starting from an initial condition, we pick all the decaying states, whereas if we watch the time evolution ending in a terminal condition, we pick all the growing states. This shows that the arrow of time is generated by the difference in the way we watch the time evolution.
References:
- G. Ordonez, N. Hatano, The arrow of time in open quantum systems and dynamical breaking of the resonance-anti-resonance symmetry, J. Phys. A: Math. Theor. 50 (2017) 405304 (34pp)
- G. Ordonez, N. Hatano, Irreversibility and the breaking of resonance-antiresonance symmetry, Chaos 27 (2017) 104608 (10pp)
- N. Hatano and G. Ordonez, Time-Reversal Symmetry and Arrow of Time in Quantum Mechanics of Open Systems, Entropy 21 (2019) 380 (14 pages)
第０６回 No. 06
講師 Speaker: 曽根和樹さん（東大）Mr. Kazuki Sone (U. Tokyo)日時 Date: １２月１４日（水）午後２時３０分〜 Wed, Oct. 9, 2:30pm
演題 Title: Non-Hermitian exceptional edge mode and its application to photonic and biological systems
要旨 Abstract: Band topology has attracted much attention due to its correspondence to the existence of robust boundary modes. Recent studies have extended the notion of topology to non-Hermitian systems. Such extension of band topology has revealed unique non-Hermitian topological phenomena such as exceptional points and non-Hermitian skin effects. In addition, non-Hermitian topological systems are abundant in both classical and quantum systems and thus have enriched the application of topological physics. In this talk, I introduce unconventional gapless modes termed as exceptional edge modes, which are unique to non-Hermitian systems. Such gapless boundary modes utilize the nontrivial topology of exceptional points and can robustly exist even with a trivial bulk. Thus, the existence of the exceptional edge modes can imply the breakdown of the bulk-boundary correspondence, while it can broaden the range of physical systems that can exhibit robust boundary modes. I also discuss the application of exceptional edge modes to topological lasers and non-Hermitian topology in active matter.