2023年度セミナー Seminars in 2023
今年度後期は月曜日午後２時から生産技術研究所（柏キャンパス）研究実験棟Ⅰの３階大会議室または１階プレゼンルームで行います。 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。
The second half of this school year, we do regular seminars from 2pm, ever Monday at the large conference roomon the third floor or the presentation roon on the first 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 |
| 5月10日(水) Wed, May 10 |
14:00 |
吉田博信さん（東大・桂研） Hironobu Yoshida (U. Tokyo) |
Liouvillian gap and single spin-flip dynamics in the dissipative Fermi-Hubbard model Abstract |
| 5月17日(水) Wed, May 17 |
14:00 |
大森祥輔さん（早大） Dr. Shosuke Omori (Waseda U.) |
Rigged Hilbert Space formulation for Non-Hermite System with Positive Definite Metric Abstract |
| 6月28日(水) Wed, June 28 |
14:00 |
龔宗平さん（東京大） Dr. Zongping Gong (University of Tokyo) |
Non-Hermitian physics, topological phases, and nanophotonics Abstract |
| 7月5日(水) Wed, July 5 |
14:00 |
Dr. Gonzalo Ordonez (Butler U., U.S.A.) | Quantum search in a hypercube lattice model Abstract |
| 7月12日(水) Wed, July 12 |
14:00 |
渡辺あかねさん（早大） Akane Watanabe (Waseda U.) |
CP condition for non-Markovian dynamics in an exactly solvable open Jaynes-Cummings model Abstract |
| 7月19日(水) Wed, July 19 |
14:00 |
Dr. Hui Wang (RIKEN) | Relational superposition measurements with a material quantum ruler Abstract |
| 10月10日(火) Tue, October 10 |
14:00 |
早川尚男さん（京大基研） Prof. Hisao Hayakawa (YITP, Kyoto U.) |
Quantum Mpemba effect: an anomalous thermal relaxation process in quantum matter Abstract |
| 10月16日(月) Mon, October 16 |
14:00 |
井村健一郎さん（東大生研） Dr. Ken-Ichiro Imura (IIS, U. Tokyo) |
From topological matter to non-Hermitian quantum mechanics Abstract |
| 10月23日(月) Mon, October 23 |
14:00 |
櫻井理人さん（埼玉大） Mr. Rihito Sakurai (Saitama U.) |
Solving differential equations for chemical kinetics using quantics tensor train Abstract |
| 10月30日(月) Mon, October 30 |
14:00 |
Dr. Paul Menczel (RIKEN) | Quantum Thermal Machines at Weak and Strong Coupling Abstract |
| 11月13日(月) Mon, November 13 |
14:00 |
石黒裕樹さん（東大物性研） Dr. Yuki Ishiguro (ISSP, U. Tokyo) |
Quasiparticles in non-Hermitian quantum integrable systems Abstract |
| 11月20日(月) Mon, November 20 |
14:00 |
牛原啓さん（東大物理） Mr. Hiromu Ushihara (Dept. Phys. U. Tokyo) |
Microscopically-derived quantum master equation for a boundary-driven Hubbard model and its application to non-linear thermoelectric effect Abstract |
| 11月27日(月) Mon, November 27 |
14:00 |
中川大也さん（東大物理） Dr. Masaya Nakagawa (Dept. Phys., U. Tokyo) |
Topology of discrete quantum feedback control Abstract |
| 12月4日(月) Mon, December 4 |
13:30 |
花井奏太さん（慶應大物理） Mr. Sota Hanai (Dept. Phys., Keio U.) |
Chiral magnetic waves in quark matter inside neutron stars and gravitational waves Abstract |
| 12月11日(月) Mon, December 11 |
14:00 |
橋本一成さん（山梨大工） Kazunari Hashimoto (Yamanashi U.) |
Signature of Liouvillian exceptional point in stationary current noise Abstract |
| 12月18日(月) Mon, December 18 |
14:00 |
Dr. Tan Van Vu (RIKEN) | Speed limits meet optimal transport: Applications in quantum many-body systems Abstract |
第０１回 No. 01
講師 Speaker: 吉田博信さん（東京大）Hironobu Yoshida (Katsura Lab., The University of Tokyo)日時 Date: ５月１０日（水）午後２時〜 Wed, May 10, 2:00pm
演題 Title: Liouvillian gap and single spin-flip dynamics in the dissipative Fermi-Hubbard model
要旨 Abstract: Recent progress in experiments on ultra-cold atoms has made it possible to realize well-controlled open many-body quantum systems, where couplings to the environment are possible resources to engineer novel non-equilibrium states. For example, the Fermi-Hubbard model with SU(N) spin symmetry is realized with alkaline-earth-like atoms, and well-controlled two-body loss can be induced by photoassociation. One counter-intuitive result observed in experiments is that in a strongly dissipative regime, losses can be suppressed by a continuous quantum Zeno effect.
Motivated by these experiments, we study the SU(N) Fermi-Hubbard model on a d-dimensional hypercubic lattice with two-body loss [1]. By focusing on states near the ferromagnetic steady states, we obtain the Liouvillian gap in closed form for any d and N. We also investigate the dynamics of a ferromagnetic initial state with a single spin flip analytically in strongly- and weakly-interacting and dissipative limits and numerically for various values of the parameters. Then we show that, by decreasing the strength of the interaction and loss, a crossover from the power-law decay to the exponential decay occurs.
参考文献 ReferencesH. Yoshida and H. Katsura, Phys. Rev. A 107 033332 (2023)
第０２回 No. 02
講師 Speaker: 大森祥輔さん（早大）Dr. Shosuke Omori (Waseda U.)日時 Date: ５月１７日（水）午後２時〜 Wed, May 17, 2:00pm
演題 Title: Rigged Hilbert Space formulation for Non-Hermite System with Positive Definite Metric
要旨 Abstract: See PDF here. 参考文献 References
S. Omori and J. Takahashi, J. Math. Phys. 63 123503 (2022)
第０３回 No. 03
講師Speaker:龔宗平さん（東京大） Dr. Zongping Gong (University of Tokyo)日時 Date: ６月２８日（水）午後２時〜 Wed, June 28, 2:00pm
演題 Title: Non-Hermitian physics, topological phases, and nanophotonics
要旨 Abstract: The recent decades have witnessed the rise of non-Hermitian physics [1]. Literally, this is a field studying physical systems or processes described by non-Hermitian matrices or operators. It is relevant to a broad class of open systems, either quantum or classical. Essential progress has been made in defining, discovering, and classifying non-Hermitian topological phases [2-4], which are the dissipative generalizations of conventional topological materials described by Hermitian Hamiltonians. Meanwhile, exploring the physics of quantum emitters in structured bath has become a central topic in (quantum) nanophotonics [5]. This topic is not only of fundamental importance in understanding how light-matter interactions can be influenced by the photonic environment, but also of increasing practical relevance in light of the rapid development on engineering various nanophotonic structures. In this seminar, I will first give a minimal tutorial on non-Hermitian physics with a special focus on topological phases. Then I will talk about how we merge the two fields by investigating the behaviors of quantum emitters in lossy nanophotonic structures with unique non-Hermitian features [6]. 参考文献 References
[1] Y. Ashida, Z. Gong, and M. Ueda, Adv. Phys. 69, 249 (2020).
[2] Z. Gong, Y. Ashida, K. Kawabata, K. Takasan, S. Higashikawa, and M. Ueda, Phys. Rev. X 8, 031079 (2018).
[3] K. Kawabata, K. Shiozaki, M. Ueda, and M. Sato, Phys. Rev. X 9, 041015 (2019).
[4] E. J. Bergholtz, J. C. Budich, and F. K. Kunst, Rev. Mod. Phys. 93, 015005 (2021).
[5] D. E. Chang, J. S. Douglas, A. González-Tudela, C.-L. Hung, and H. J. Kimble, Rev. Mod. Phys. 90, 031002 (2018).
[6] Z. Gong, M. Bello, D. Malz, and F. K. Kunst, Phys. Rev. Lett. 129, 223601 (2022); Phys. Rev. A 106, 053517 (2022).
第０４回 No. 04
講師Speaker:Dr. Gonzalo Ordonez (Butler University, U.S.A.)日時 Date: ７月５日（水）午後２時〜 Wed, July 5, 2:00pm
場所 Place: 東京大学生産技術研究所研究実験棟１（柏キャンパス）大会議室 The University of Tokyo, Institute of Industrial Science, Research and Testing Complex I (Kashiwa campus) Large Conference Room
演題 Title: Quantum search in a hypercube lattice model
要旨 Abstract: Quantum lattice models can be used to analyze many physical systems, such as semiconductor quantum wires, carbon nanotubes or topological insulators. Here we consider a tight-binding model of a hypercubic lattice in n dimensions, which correspond to the space of n-qubit words. The quantum propagation of a single particle in this lattice can be related to the search of an n-qubit word. We show that the time it takes to find an n-qubit word using quantum propagation is proportional to √(2^n ), whereas a classical search would take a time proportional to 2^n. We discuss an implementation this model in an IBM quantum computer and compare results with Grover’s quantum search algorithm, which is one of the best-known quantum search algorithms.
第０５回 No. 05
講師Speaker:渡辺あかねさん（早大） Akane Watanabe (Waseda U.)日時 Date: ７月１２日（水）午後２時〜 Wed, July 12, 2:00pm
場所 Place: 東京大学生産技術研究所研究実験棟１（柏キャンパス）大会議室 The University of Tokyo, Institute of Industrial Science, Research and Testing Complex I (Kashiwa campus) Large Conference Room
演題 Title: CP condition for non-Markovian dynamics in an exactly solvable open Jaynes-Cummings model
要旨 Abstract: Master equations of the GKSL form describe Markovian dynamics of open quantum systems and always preserve CPTP. Other master equations could describe non-Markovian dynamics, but their preservation of CPTP is generally difficult to verify. We propose a solvable open Jaynes-Cummings model to treat dynamics beyond that described by the GKSL form. Our assumptions include the initial uncorrelation and the bosonic bath initially being thermal equilibrium. No other conditions are imposed in the course of solving the dynamics. Specifically, all orders of the interaction between the system and the environment are taken into account, and the time evolution of the environment affects the dynamics of the system. We solved the exact dynamics of the system and obtained its Kraus representation, which rigorously yields the corresponding CP condition. We also obtained the exact master equation in the GKSL-like form, with time-dependent damping rates. Our result is a concrete example of the CP condition for non-Markovian dynamics and gives an insight into the general case. 参考文献 References
A. Watanabe and H. Nakazato, Ann. Phys. 441, 168890 (2022).
第０６回 No. 06
講師Speaker:Dr. Hui Wang (RIKEN)日時 Date: ７月１９日（水）午後２時〜 Wed, July 19, 2:00pm
場所 Place: 東京大学生産技術研究所研究実験棟１（柏キャンパス）大会議室 The University of Tokyo, Institute of Industrial Science, Research and Testing Complex I (Kashiwa campus) Large Conference Room
演題 Title: Relational superposition measurements with a material quantum ruler
要旨 Abstract: In physics, it is crucial to identify operational measurement procedures to give physical meaning to abstract quantities. There has been significant effort to define time operationally using quantum systems, but the same has not been achieved for space. Developing an operational procedure to obtain information about the location of a quantum system is particularly important for a theory combining general relativity and quantum theory, which cannot rest on the classical notion of spacetime. Here, we take a first step towards this goal, and introduce a model to describe an extended material quantum system working as a position measurement device. Such a “quantum ruler” is composed of N harmonically interacting dipoles and serves as a (quantum) reference system for the position of another quantum system. We show that we can define a quantum measurement procedure corresponding to the “superposition of positions”, and that by performing this measurement we can distinguish when the quantum system is in a coherent or incoherent superposition in the position basis. The model is fully relational, because the only meaningful variables are the relative positions between the ruler and the system, and the measurement is expressed in terms of an interaction between the measurement device and the measured system.
第０７回 No. 07
講師Speaker:早川尚男さん（京大基研）Prof. Hisao Hayakawa (Yukawa Institute, Kyoto University)日時 Date: １０月１０日（火）午後２時〜 Tuesday, October 10, 2:00pm
場所 Place: 東京大学生産技術研究所研究実験棟１（柏キャンパス）大会議室 The University of Tokyo, Institute of Industrial Science, Research and Testing Complex I (Kashiwa campus) Large Conference Room
演題 Title: Quantum Mpemba effect: an anomalous thermal relaxation process in quantum matter
要旨 Abstract: Mpemba effect refers to the counter-intuitive phenomenon where a hotter object can cool down faster than a colder copy of the same object. In spite of some theoretical as well as experimental advances in the classical domain, the quantum counterpart of the Mpemba effect, specifically in temperature, has remained unexplored. In this talk, we demonstrate the quantum Mpemba effect by showing that temperatures of two copies of a quantum system, one initially hotter than the other, can cross each other after some time and thereafter reverse their identities, i.e. hotter becomes colder and vice versa, before reaching the same final temperature. In fact, we show such crossing of trajectories characterizing the quantum Mpemba effect, which can occur in several other observables including energy, entropy etc. Our theoretical results on the quantum Mpemba effect are primarily based on a quantum dot connected to two reservoirs [1]. In the later part of the talk, we discuss how exceptional points and complex eigenvalue spectrum can lead to multiple quantum Mpemba effect (where crossing occurs multiple times) in a two-level driven dissipative system proposed by Hatano [2].
Reference:
[1] A. K. Chatterjee, S. Takada, and H. Hayakawa, Phys. Rev. Lett. 131, 080402 (2023).
[2] N. Hatano, Mol. Phys. 117, 2121 (2019).
第０８回 No. 08
講師Speaker:井村健一郎さん（東大生研）Ken-Ichiro Imura (Institute of Industrial Science, the University of Tokyo)日時 Date: １０月１６日（月）午後２時〜 Monday, October 16, 2:00pm
場所 Place: 東京大学生産技術研究所研究実験棟１（柏キャンパス）大会議室 The University of Tokyo, Institute of Industrial Science, Research and Testing Complex I (Kashiwa campus) Large Conference Room
演題 Title: From topological matter to non-Hermitian quantum mechanics
要旨 Abstract: Topological properties of topological matter are protected by the topology of the wave function, which does no change unless the system is perturbed so strongly that the bulk energy gap closes (as a result, related bands get inverted). In ref. [1], using such a protected topological property, we have proposed a recipe for realizing an ultra-low-energy consuming nanocircuit on terraces of a weak topological insulator. In refs. [2-3], topological insulator and Weyl semimetal thin (nano-) films have been studied from the viewpoint of the dimensional crossover of topological properties in these systems. In ref. [4] the idea of topological insulator has been generalized to non-Hermitian systems. After reviewing these works on topological matter, I will discuss in the body of the talk, our recent work [5] on the entanglement dynamics in a (Hatano-Nelson type [6]) non-Hermitian system.
[1] Yukinori Yoshimura, Akihiko Matsumoto, Yositake Takane, and Ken-Ichiro Imura, Phys. Rev. B 88, 045408 (2013)
[2] Koji Kobayashi, Yukinori Yoshimura, Ken-Ichiro Imura, and Tomi Ohtsuki, Phys. Rev. B 92, 235407 (2015)
[3] Yukinori Yoshimura, Wataru Onishi, Koji Kobayashi, Tomi Ohtsuki, and Ken-Ichiro Imura, Phys. Rev. B 94, 235414 (2016)
[4] Ken-Ichiro Imura and Yositake Takane, Phys. Rev. B 100, 165430 (2019)
[5] Takahiro Orito, Ken-Ichiro Imura, arXiv:230803078
[6] N. Hatano, D.R. Nelson, Phys. Rev. Lett. 77, 570 (1996)
第０９回 No. 09
講師Speaker:櫻井理人さん（埼玉大学） Rihito Sakurai (Saitama U.)日時 Date: １０月２３日（月）午後２時〜 Monday, October 23, 2:00pm
場所 Place: （外部非公開です Not open to public.）
演題 Title: Solving differential equations for chemical kinetics using quantics tensor train
要旨 Abstract:
In a chemical system, multiple elementary reactions, each with significantly different reaction rates, often occur. In such systems, when solving differential equations numerically, the time step of numerical simulations is confined to a very small time scale, resulting in a substantial increase in the computational cost. Quantics tensor train (QTT), a type of tensor network, can adaptively compress a scalar-valued function according to the amount of correlation between different length scales [1, 2, 3]. This study considers compression in the QTT format of nonlinear ordinary differential equations for chemical kinetics models. In this seminar, I will start with an introduction to tensor networks. Then, I will talk about two preliminary results: verifying a low-rank structure for a kinetics model of Escherichia coli core metabolism [4] and an iterative method for solving the nonlinear ordinary differential equations.
[1] I. V. Oseledets, Doklady Math. 80, 653 (2009)
[2] B. N. Khoromskij, Constr. Approx. 34, 257 (2011)
[3] H. Shinaoka, M. Wallerberger, Y. Murakami, K. Nogaki, R. Sakurai, P. Werner, A. Kauch, Phys. Rev. X 13, 021015 (2023)
[4] A. Khobayari, A. R. Zomorrodi, J.C. Liao, C. D. Maranas, Metabolic Engineering 25, 50 (2014)
第１０回 No. 10
講師Speaker:Dr. Paul Menczel (RIKEN)日時 Date: １０月３０日（月）午後２時〜 Monday, October 30, 2:00pm
場所 Place: 東京大学生産技術研究所研究実験棟１（柏キャンパス）大会議室 The University of Tokyo, Institute of Industrial Science, Research and Testing Complex I (Kashiwa campus) Large Conference Room
演題 Title: Quantum Thermal Machines at Weak and Strong Coupling
要旨 Abstract:
A thermal machine is a device that operates by controlling the thermal energy of its environment. Experimental labs are nowadays building thermal machines that consist of as little as a single atom, and quantum effects are beginning to influence their performance. Can quantum effects be exploited to gain an advantage compared to traditional devices? To approach this question, one first needs to understand the dynamics of non-isolated (i.e., open) quantum systems. I will therefore begin this seminar with a brief introduction to modern techniques for working with open quantum systems. Since the answer to the introductory question heavily depends on the strength of the coupling between the quantum system and its environment, I will focus in particular on the more difficult strong-coupling regime. I will then return to the topic of quantum thermodynamics, discussing quantum thermal machines and ways to assess their performance.
第１１回 No. 11
講師Speaker:石黒裕樹さん（東京大学物性研） Yuki Ishiguro (ISSP, U. Tokyo)日時 Date: １１月１３日（月）午後２時〜 Monday, November 13, 2:00pm
場所 Place: 東京大学生産技術研究所研究実験棟１（柏キャンパス）大会議室 The University of Tokyo, Institute of Industrial Science, Research and Testing Complex I (Kashiwa campus) Large Conference Room
演題 Title: Quasiparticles in non-Hermitian quantum integrable systems
要旨 Abstract: Quantum integrable systems, which are exactly solvable through the Bethe ansatz, play crucial roles in understanding many-body systems. While there are known examples of integrable systems in non-Hermitian cases, the theoretical framework for their analysis is not as well-developed as it is for Hermitian systems. In the Bethe ansatz, the problem of diagonalizing the Hamiltonian is transformed into solving the Bethe equations. Analyzing the Bethe equations is generally challenging; however, in many cases, it can be achieved by considering the structure of quasiparticles.
In this talk, we will discuss quasiparticles in non-Hermitian quantum integrable systems. First, we will introduce the basics of the Bethe ansatz and illustrate its utility by presenting our recent work on quantum solitons as an example, where we computed exact dynamics in quantum integrable systems [1]. Then, we will explain string solutions, specific solutions to the Bethe equations that are widely observed in various quantum integrable systems. String solutions correspond to certain types of quasiparticles closely related to solitons in classical integrable systems and integrable cellular automata and allow the analysis of the Bethe equations through the so-called string hypothesis. Through an investigation of the Bethe equation for the asymmetric simple exclusion process, an integrable spin chain extended to a non-Hermitian system by introducing asymmetry into the Heisenberg model, we will show how the quasiparticles picture in quantum integrable systems is changed by the non-Hermiticity [2].
[1] Yuki Ishiguro, Jun Sato, Takahiro Ezaki, and Katsuhiro Nishinari, “Constructing quantum dark solitons with stable scattering properties”, Phys. Rev. Research 4, L032047 (2022)
[2] Yuki Ishiguro, Jun Sato, and Katsuhiro Nishinari, “Asymmetry-induced delocalization transition in the integrable non-Hermitian spin chain”, Phys. Rev. Research 5, 033102 (2023)
第１２回 No. 12
講師Speaker:牛原啓さん（東大物理）Mr. Hiromu Ushihara (Dept. Phys. U. Tokyo)
日時 Date: １１月２０日（月）午後２時〜 Monday, November 20, 2:00pm
場所 Place: 東京大学生産技術研究所研究実験棟１（柏キャンパス）大会議室 The University of Tokyo, Institute of Industrial Science, Research and Testing Complex I (Kashiwa campus) Large Conference Room
演題 Title: Microscopically-derived quantum master equation for a boundary-driven Hubbard model and its application to non-linear thermoelectric effect
要旨 Abstract: Boundary-driven systems, systems coupled to two external baths at their ends, are good platforms for studying nonequilibrium states and transport phenomena. One way to describe quantum systems in contact with external degrees of freedom is using quantum master equations in the theory of open quantum systems [1]. Quantum master equations are well understood for small open systems, such as atoms or molecules coupled to environments. However, it has been claimed that the established equations are not directly applicable to open many-body systems or transport phenomena [2].
In this study, we derive a quantum master equation describing a boundary-driven fermionic Hubbard model, and numerically verify its applicability to studies of transport phenomena [3]. We apply the derived equation to thermoelectric effect and find a non-trivial nonlinear behavior through numerical calculations. In the seminar, I will begin with a brief introduction to quantum master equations and their issues. Then, I will give a brief sketch of our derivation of the equation, and explain numerical results. In particular, the nonlinear thermoelectric effect and the physical picture based on the derived equation will be discussed.
[1] H.-P. Breuer and F. Petruccione, The Theory of Open Quantum Systems (Oxford University Press, 2007).
[2] H. Wichterich, M. J. Henrich, H.-P. Breuer, J. Gemmer, and M. Michel, Phys. Rev. E 76, 031115 (2007).
[3] H. Ushihara, K. Takasan, and N. Tsuji, in preparation.
第１３回 No. 13
講師Speaker:中川大也さん（東京大学物理） Masaya Nakagawa (Dept. Phys. U. Tokyo)>日時 Date: １１月２７日（月）午後２時〜 Monday, November 27, 2:00pm
場所 Place: 東京大学生産技術研究所研究実験棟１（柏キャンパス）大会議室 The University of Tokyo, Institute of Industrial Science, Research and Testing Complex I (Kashiwa campus) Large Conference Room
演題 Title: Topology of discrete quantum feedback control
要旨 Abstract: Abstract: Symmetry and topology are guiding principles for the classification of quantum phases of matter. For instance, unique gapped ground states of quantum many-body systems are classified into symmetry-protected topological phases, where different phases cannot be continuously connected with each other while preserving symmetry and an energy gap [1]. One can also utilize symmetry and topology to classify nonequilibrium dynamics. Examples include Floquet topological phases with topological unitary operators [2] and non-Hermitian topological phases in open systems [3]. In this seminar, we develop a framework of a topological classification of quantum channels that describe discrete quantum feedback control [4]. We construct topological Maxwell's demon that achieves robust feedback-assisted chiral transport due to its topological nature. Furthermore, we provide a symmetry classification of quantum feedback control and show that only ten symmetry classes out of the 38-fold Bernard-LeClair classes are consistent with projective measurement. Building on the symmetry classification, we construct symmetry-protected topological feedback control which exhibits helical spin transport due to nontrivial point-gap topology.
[1] X. Chen, Z. C. Gu, Z. X. Liu, and X. G. Wen, Phys. Rev. B 87, 155114 (2013).
[2] T. Kitagawa, E. Berg, M. Rudner, and E. Demler, Phys. Rev. B 82, 235114 (2010).
[3] Z. Gong, Y. Ashida, K. Kawabata, K. Takasan, S. Higashikawa, and M. Ueda, Phys. Rev. X 8, 031079 (2018).
[4] M. Nakagawa and M. Ueda, in preparation.
第１４回 No. 14
講師Speaker:花井奏太さん（慶應大物理） Sota Hanai (Dept. Phys. Keio U.)日時 Date: １２月４日（月）午後１時半〜 Monday, December 4, 1:30pm
＊＊通常より早く始まります。Note that the seminar starts earlier than usual.
場所 Place: 東京大学生産技術研究所研究実験棟１（柏キャンパス）大会議室 The University of Tokyo, Institute of Industrial Science, Research and Testing Complex I (Kashiwa campus) Large Conference Room
演題 Title: Chiral magnetic waves in quark matter inside neutron stars and gravitational waves
要旨 Abstract: It is important to unravel the internal structure of neutron stars in astrophysics. One effective way to study the interior of neutron stars is analyzing their seismic oscillations. Recently, the chiral magnetic wave (CMW), which is a density wave propagating along magnetic fields due to the chirality of fermions, has been studied in the context of the heavy ion collision experiments. In this talk, we show that the CMW can appear as a seismic oscillation in quark matter, such as the two-flavor color superconductivity, inside neutron stars. We also discuss the frequency and amplitude of a new type of gravitational wave radiated by the seismic oscillation. This gravitational wave could be a new probe of the magnetic field and quark matter in neutron stars.
第１５回 No. 15
講師Speaker:橋本一成さん（山梨大工） Kazunari Hashimoto (Yamanashi U.)日時 Date: １２日１１日（月）午後２時〜 Monday, December 11 2:00pm
場所 Place: 東京大学生産技術研究所研究実験棟１（柏キャンパス）大会議室 The University of Tokyo, Institute of Industrial Science, Research and Testing Complex I (Kashiwa campus) Large Conference Room
演題 Title: Signature of Liouvillian exceptional point in stationary current noise
要旨 Abstract: Open quantum systems coupled to thermal reservoirs naturally exhibit non-Hermitian physics; their time evolution can be described by quantum master equations characterized by Liouvillian superoperators, accounting for both free Hamiltonian evolution and dissipation due to coupling to the reservoirs, the latter being inherently non-Hermitian. An interesting feature of non-Hermitian physics is the presence of exceptional points (EPs), which can also be found in dissipative open quantum systems as Liouvillian EPs [1]. At an EP, the operator exhibits a singularity where eigenvalues and corresponding eigenvectors coincide, rendering the operator non-diagonalizable and only transformable into the Jordan block form.
Investigating physical signatures of Hamiltonian and Liouvillian EPs in state-of-the-art physical platforms, particularly in the quantum regime, is an active research area. A recent study on Liouvillian EPs in a quantum thermal machine [2], consisting of two interacting qubits each in contact with its thermal reservoir, revealed that the dynamical signature manifests in the transient relaxation process as critical decay towards the steady state.
In the present study, I explore an alternative approach to capturing physical signatures of Liouvillian EPs in quantum thermal machines, focusing on the steady-state noise of the electronic current between two thermal reservoirs. By formulating the power spectrum of the two-time autocorrelation function of the current using full counting statistics (FCS), I analyzed the stationary current noise at Liouvillian EPs. As a result, I found that the Jordan block structure at Liouvillian EPs provides a super-Lorentzian line shape of the noise spectrum.
[1] N. Hatano, Molecular Phys. 117, 2121(2019).
[2] S. Khandelwal, N. Brunner, and G. Haack, PRX Quantum 2, 040346(2021).
第１６回 No. 16
講師Speaker:Dr. Tan Van Vu (RIKEN)日時 Date: １２日１８日（月）午後２時〜 Monday, December 18 2:00pm
場所 Place: 東京大学生産技術研究所研究実験棟１（柏キャンパス）大会議室 The University of Tokyo, Institute of Industrial Science, Research and Testing Complex I (Kashiwa campus) Large Conference Room
演題 Title: Speed limits meet optimal transport: Applications in quantum many-body systems
要旨 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 can be traced back to the seminal work by Mandelstam and Tamm [1], where they established a constraint on the minimum time required for transitions between orthogonal states in closed quantum dynamics. This work led to the development of the concept of quantum speed limits, which has found applications in various areas of physics. The second avenue of research emerged with Lieb and Robinson’s pioneering work [2], which primarily focused on the speed at which information propagates in quantum spin systems. They introduced the notion of an effective light cone, beyond which information propagation exponentially decays with increasing distance. This concept, known as the Lieb-Robinson bound, has proven to be a powerful tool for analyzing quantum many-body systems and has found applications in diverse fields. These two speed-limit concepts have evolved independently, each with its strengths and weaknesses.
In this seminar, I will present a novel speed-limit framework based on optimal transport theory, which is a mature field in mathematics that concerns the optimal planning and optimal cost of transporting a distribution. First, I will briefly explain the two notions of speed limits mentioned above and the theory of optimal transport with particular attention to the Wasserstein distance. Then, I will describe a unified speed limit, derived from optimal transport theory, which can be applied to a broad spectrum of dynamics. This unified speed limit not only leads to a topological speed limit that takes the geometrical structure of dynamics into account [3] but also can address a long-standing unresolved issue related to bosonic transport [4].
[1] L. Mandelstam and I. Tamm, J. Phys. USSR 9, 249 (1945).
[2] E. H. Lieb and D. W. Robinson, Commun. Math. Phys. 28, 251 (1972).
[3] T. Van Vu, K. Saito, Phys. Rev. Lett. 130, 010402 (2023).
[4] T. Van Vu, T. Kuwahara, K. Saito, arXiv:2307.01059.