イベント検索

Nonlinear response in strongly correlated systems

2021年10月20日(水) 17:00 - 18:15

Robert Peters (京都大学 大学院理学研究科 講師)

Nonlinear responses in condensed matter are intensively studied because they provide rich information about materials and hold the possibility of being applied in diodes or high-frequency optical devices [1-4]. While nonlinear responses in noninteracting models have been explored widely, the effect of strong correlations on the nonlinear response is still poorly understood. This talk will introduce a Green's function method to calculate nonlinear conductivities in strongly correlated materials [5-6]. Correlation effects are thereby included by the self-energy of the material. I will then use this method to study the nonlinear conductivities in noncentrosymmetric f-electron systems. The first system is a heavy Fermion system, where a nonreciprocal conductivity appears in the ferromagnetic phase. The nonreciprocal conductivity thereby always occurs perpendicular to the magnetization of the system and has a strong spin dependence, which might be advantageous for spintronic applications. The second system is a model corresponding to the Weyl-Kondo semimetal Ce3Bi4Pd3, in which a giant spontaneous Hall effect without time-reversal symmetry breaking has been observed [7]. This Hall effect can be explained as a nonlinear Hall effect in an inversion-symmetry broken Weyl-semimetal. It has been shown that the nonlinear Hall effect is related to the Berry curvature dipole [4]. Our study shows that the magnitude of the experimentally observed nonlinear Hall effect can be explained by the strong correlations inherent in this f-electron material [8]. *Detailed information about the seminar refer to the email.

会場: via Zoom

イベント公式言語: 英語

Symmetry-based analysis for unconventional superconductors: Diagnosis of topological and nodal superconductivity

2021年10月12日(火) 16:00 - 17:15

小野 清志郎 (東京大学 工学系研究科 物理工学専攻)

The physics of unconventional superconductors has gained a new dimension in the past decade, thanks to the bloom in the understanding of topological quantum materials. Keeping in mind the success of the symmetry-based diagnosis in the large-scale discovery of topological insulator and semimetal candidates [1], it is natural to ask whether the approach can be generalized to superconducting systems. In this talk, I provide a unified way to diagnose topology and superconducting nodes in unconventional superconductors. First, I review symmetry-indicator theory for the topological insulators [2]. Also, I also discuss how to generalize the theory to superconductors [3,4,5]. Next, I show that the symmetry-based approach can extensively classify superconducting nodes pinned to high-symmetric momenta [6]. Finally, I show that these results enable us to derive the comprehensive correspondences between pairing symmetries and topological/nodal superconducting nature for each material [7].
 *Detailed information about the seminar refer to the email.

会場: via Zoom

イベント公式言語: 英語

Wave function geometry and anomalous Landau levels of flat bands

2021年10月7日(木) 16:00 - 17:15

Bohm-Jung Yang (Associate Professor, Department of Physics and Astronomy, Seoul National University, Republic of Korea)

Semiclassical quantization of electronic states under magnetic field describes not only the Landau level spectrum but also the geometric responses of metals under a magnetic field. However, it is unclear whether this semiclassical idea is valid in dispersionless flat-band systems, in which an infinite number of degenerate semiclassical orbits are allowed. In this talk, I am going to show that the semiclassical quantization rule breaks down for a class of flat bands including singular flat bands [1-5] and isolated flat bands [6]. The Landau levels of such a flat band develop in the empty region in which no electronic states exist in the absence of a magnetic field. The total energy spread of the Landau levels of flat bands is determined by the quantum geometry of the relevant Bloch states, which is characterized by their Hilbert–Schmidt quantum distance and fidelity tensors. The results indicate that flat band systems are promising platforms for the direct measurement of the quantum geometry of wavefunctions in condensed matter. *Detailed information about the seminar refer to the email.

会場: via Zoom

イベント公式言語: 英語

Speed limit for population dynamics

2021年10月7日(木) 10:00 - 11:00

足立 景亮 (理化学研究所 生命機能科学研究センター (BDR) 生体非平衡物理学理研白眉研究チーム 基礎科学特別研究員)

In statistical and quantum physics, the speed limit, i.e., upper bound for change rate, of time-dependent quantities has been discussed. In this talk, I will extend the concept of speed limit to ecological and evolutionary processes by considering the competitive Lotka-Volterra model and the quasi-species model. As an application of the speed limit, I will also discuss the universal constraint for the relaxation at the bifurcation point of such models.

会場: via Zoom

イベント公式言語: 英語

Finding Gravitational Waves from the Early Universe

2021年9月27日(月) 16:00 - 17:30

小松 英一郎 (Director, Max Planck Institute for Astrophysics, Germany)

The Cosmic Microwave Background (CMB) gives a photographic image of the Universe when it was still an “infant”. We have been using it to test our ideas about the origin of the Universe. The CMB research told us a remarkable story: the structure we see in our Universe such as galaxies, stars, planets, and eventually ourselves originated from tiny quantum fluctuations in the period of the early Universe called cosmic inflation. While we have accumulated strong evidence for this picture, the extraordinary claim requires extraordinary evidence. The last prediction of inflation that is yet to be confirmed is the existence of primordial gravitational waves whose wavelength can be as big as billions of light years. To this end we have proposed to JAXA a new satellite mission called LiteBIRD, whose primary scientific goal is to find signatures of gravitational waves in the polarisation of the CMB. In this presentation we describe physics of gravitational waves from inflation, and the LiteBIRD proposal.

会場: via Zoom

イベント公式言語: 英語

S-matrix Unitarity toward UV Completion

2021年9月13日(月) 13:30 - 15:00

泉 圭介 (名古屋大学 素粒子宇宙起源研究所 (KMI) 助教)

Einstein gravity is not renormalizable and does not hold perturbative unitarity at high energy. This is the main reason why the construction of quantum gravity is difficult. A conjecture was proposed by Llewellyn Smith, "renormalizablility and tree-unitarity at high energy give the same conditions for theories". This conjecture would be important because it shows that, if a theory is constructed s.t. unitarity is satisfied, renormalizablility holds automatically, and vice versa. Unfortunately, a counterexample was pointed out. If a theory involves higher derivatives, there exists a theory which is renormalizable but does not satisfy tree-unitarity. A candidate of quantum gravity, the quadratic gravity (R_{\mu\nu}^2 gravity), is one of the examples. Therefore, Llewellyn Smith's conjecture would not be useful for the discussion of quantum gravity. Then, we introduce a new conjecture, "renormalizablility and S-matrix unitarity (or often called pseudo-unitarity) at high energy give the same conditions for theories". In this talk, Llewellyn Smith's conjecture and our contribution to it will be explained. Then, our new conjecture will be introduced. Finally, it will be shown that our conjecture works well even in theories with higher derivatives.

会場: via Zoom

イベント公式言語: 英語

Extended and interacting bound states in elemental superconductors

2021年9月1日(水) 16:00 - 17:15

Levente Rózsa (Condensed Matter Physics, University of Konstanz, Germany)

Combining magnetism with superconductivity leads to the emergence of localized states, including Majorana bound states predicted to be relevant for topological quantum computation. In this talk, I discuss how these bound states are influenced by the details of the electronic structure. It will be shown how the shape of the Fermi surface leads to a long-ranged anisotropic extension of Yu-Shiba-Rusinov states in the vicinity of magnetic impurities [1]. The same type of Fermi surface will be demonstrated to give rise to topologically trivial Caroli-de Gennes-Matricon bound states in vortex cores [2], with similar spatial profiles to those of topological Majorana bound states. The role of spin-orbit coupling will be discussed in the hybridization of Yu-Shiba-Rusinov bound states of dimers with ferromagnetic and antiferromagnetic spin alignments [3]. The general theoretical concepts will be illustrated by experimental realizations in specific materials. *Detailed information about the seminar refer to the email.

会場: via Zoom

イベント公式言語: 英語

RIKEN-Vancouver Joint Workshop on Quantum Computing

2021年8月24日(火) - 25日(水)

The main aim of this workshop is that the quantum people in RIKEN (iTHEMS and RQC) and Vancouver (Quantum BC) get together online to discuss scientific activities and explore future collaborations. Program: August 24, 2021 (8:30am - 1:00pm) Tokyo August 23, 2021 (4:30pm - 9:00pm) Vancouver Tetsuo Hatsuda (iTHEMS): Welcome + iTHEMS overview Yasunobu Nakamura (RQC): RQC overview Lukas Chrostowski (UBC): Quantum BC overview Shunji Matsuura (1QBit): Accurate state preparation on noisy quantum devices Olivia Di Matteo (UBC): Operational, gauge-free quantum tomography Yasunobu Nakamura (RQC): Towards superconducting quantum computing Jason Chang (iTHEMS): Improving Schroedinger equation implementations with gray code for adiabatic quantum computers Robert Raussendorf (UBC): Computationally universal phase of quantum matter Akira Furusawa (RQC): Large-scale quantum computing with quantum teleportation August 25, 2021 (8:30am - 1:30pm) Tokyo August 24, 2021 (4:30pm - 9:30am) Vancouver Etsuko Itou (iTHEMS): Digital quantum simulation for screening and confinement in gauge theory with a topological term Joe Salfi (UBC): Engineering long coherence times of spin-orbit qubits in silicon Seiji Yunoki (RQC): Quantum simulations for quantum many-body systems: Variational quantum algorithms and beyond Takumi Doi (iTHEMS): Hybrid quantum annealing via molecular dynamics Drew Potter (UBC): Simulating highly-entangled matter with quantum tensor networks Seigo Tarucha (RQC): High-fidelity quantum gates in silicon quantum computing Organizing Institutes: iTHEMS: RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program RQC: RIKEN Center for Quantum Computing Quantum BC Organizers: Tetsuo Hatsuda (iTHEMS) Yasunobu Nakamura (RQC) Shunji Matsuura (1QBit) Joseph Salfi (UBC) Erika Kawakami (RQC / RIKEN CPR) Neill Lambert (RIKEN CPR)

会場: via Zoom

イベント公式言語: 英語

Application of Machine Learning on Many-Body Problems

2021年8月23日(月) 16:00 - 17:15

Daw-Wei Wang (Professor, Department of Physics, National Tsinghua University, Taiwan)

Time: 4pm ~ 5:15pm (JST); 9am ~ 10:15am (CET); 3pm ~ 4:15pm (Taiwan) In this talk, I will briefly introduce the application of machine learning methods on quantum many-body problems. It includes a self-supervised learning approach to decide the topological phase transition in the systems of ultracold atoms by using Time-of-Flight images only without knowing any priori knowledge [1]. We then develop the Random Sampling Neural Networks for the investigation of quantum many body ground state properties in the strong interacting regime by a model rtained in the weak interacting regime [2]. Finally, we provide an Quantum-Inspired-Recurrent Neural Network, which could give a precise long-time dynamics of a quantum many-body system, even the model is trained in the short-time regime. We hope to show the great possibility to use machine learning as a new tool to investigate the quantum many-body problems. *Detailed information about the seminar refer to the email.

会場: via Zoom

イベント公式言語: 英語

Quantum annealing and its fundamental aspects/ Quantum annealing and its application to real world

2021年8月4日(水) 13:30 - 16:00

大関 真之 (東北大学 大学院 情報科学研究科 教授 / 東京工業大学 科学技術創成研究院 教授 / 株式会社シグマアイ 代表取締役)

Talk A (13:30~14:30) Title: Quantum annealing and its fundamental aspects Abstract: We introduce a heuristic solver for combinatorial optimization problem, quantum annealing. The quantum annealing utilizes the quantum tunneling effect to search the ground state. In particular, the Ising model with the transverse field is employed for demonstration of the quantum annealing. Most of the combinatorial optimization problem can be described by the Ising model and they are solved by quantum annealing. A decade ago, the D-Wave systems Inc. succeeded in realizing the quantum annealing in their manufactured spin system. In this talk, the concept of quantum annealing and its implementation in the D-Wave quantum annealer are introduced. Talk B (14:40~15:40) Title: Quantum annealing and its application to real world Abstract: In this talk, we review the fundamental aspects of quantum annealing and show several applications to practical combinatorial optimization problems. In particular, in Japan, many researchers in industry are interested in practical applications of quantum annealing. We, Tohoku University, are performing various collaboration with many companies in Japan. The first example is to control automated guided vehicles in collaboration with DENSO. The second one is to list hotel recommendation on a web site with Recruit lifestyle. Other ones are also exhibited as far as possible. Let us discuss a future perspective of the quantum annealing in practical applications.

会場: via Zoom

イベント公式言語: 英語

Overview of Tensor Networks in Machine Learning

2021年7月28日(水) 13:30 - 14:50

チビン・チョウ (理化学研究所 革新知能統合研究センター (AIP) テンソル学習チーム チームリーダー)

Tensor Networks (TNs) are factorizations of high dimensional tensors into networks of many low-dimensional tensors, which have been studied in quantum physics, high-performance computing, and applied mathematics. In recent years, TNs have been increasingly investigated and applied to machine learning and signal processing, due to its significant advances in handling large-scale and high-dimensional problems, model compression in deep neural networks, and efficient computations for learning algorithms. This talk aims to present a broad overview of recent progress of TNs technology applied to machine learning from perspectives of basic principle and algorithms, novel approaches in unsupervised learning, tensor completion, multi-task, multi-model learning and various applications in DNN, CNN, RNN and etc. We also discuss the future research directions and new trend in this area.

会場: via Zoom

イベント公式言語: 英語

Non-Unitary TQFTs from 3d N=4 Rank-0 SCFTs

2021年7月5日(月) 13:00 - 14:30

Myungbo Shim (Kyung Hee University, Republic of Korea)

We propose a novel procedure of assigning a pair of non-unitary topological quantum field theories (TQFTs), TFT_\pm[T_0], to a (2+1)D interacting N=4 superconformal field theory (SCFT) T_0 of rank 0, i.e. having no Coulomb and Higgs branches. The topological theories arise from particular degenerate limits of the SCFT. Modular data of the non-unitary TQFTs are extracted from the supersymmetric partition functions in the degenerate limits. As a non-trivial dictionary, we propose that F = max{ -log |S^{(+)}_{0\alpha}| } = max{ -log |S^{(-)}_{0\alpha}| }, where F is the round three-sphere free energy of T_0 and S^{(\pm)}_{0\alpha} is the first column in the modular S-matrix of TFT_\pm. From the dictionary, we derive the lower bound on F, F > -log(\sqrt{(5-\sqrt{5})/10}) \simeq 0.642965, which holds for any rank 0 SCFT. The bound is saturated by the minimal N=4 SCFT proposed by Gang-Yamazaki, whose associated topological theories are both the Lee-Yang TQFT. We explicitly work out the (rank 0 SCFT)/(non-unitary TQFTs) correspondence for infinitely many examples. Before going to the technical part, we provide some background materials including some peculiar features in 3d gauge theories, some supersymmetries, anyons, and some modular data of MTC in this talk.

会場: via Zoom

イベント公式言語: 英語

Black Hole Information Paradox and Wormholes

2021年6月21日(月) 13:00 - 14:30

後藤 郁夏人 (理化学研究所 数理創造プログラム (iTHEMS) 基礎科学特別研究員)

In this talk, I will explain about the recent progress in the black hole information paradox that I am involved with. The information paradox arises when a black hole evaporates by emitting Hawking radiation due to the quantum effects. Time dependence of the entropy of Hawking radiation is diagnosis of information loss caused by the black hole evaporation. If information is not lost, the entropy of Hawking radiation should obey the so-called Page curve. In recent research developments, it was found that “the quantum extremal islands” reproduce the unitary Page curve in an evaporating black hole. I will argue about how the quantum extremal islands are derived from the computation of the entropy of Hawking radiation using the gravitational path-integral.

会場: via Zoom

イベント公式言語: 英語

Theory of Anomalous Floquet Higher-Order Topology

2021年5月26日(水) 22:00 - 23:15

Rui-Xing Zhang (University of Maryland, College Park, USA)

Periodically-driven or Floquet systems can realize anomalous topological phenomena that do not exist in any equilibrium states of matter, whose classification and characterization require new theoretical ideas that are beyond the well-established paradigm of static topological phases. In this work, we provide a general framework to understand anomalous Floquet higher-order topological insulators (AFHOTIs), the classification of which has remained a challenging open question. In two dimensions (2D), such AFHOTIs are defined by their robust, symmetry-protected corner modes pinned at special quasienergies, even though all their Floquet bands feature trivial band topology. The corner-mode physics of an AFHOTI is found to be generically indicated by 3D Dirac/Weyl-like topological singularities living in the phase spectrum of the bulk time-evolution operator. Physically, such a phase-band singularity is essentially a "footprint" of the topological quantum criticality, which separates an AFHOTI from a trivial phase adiabatically connected to a static limit. Strikingly, these singularities feature unconventional dispersion relations that cannot be achieved on any static lattice in 3D, which, nevertheless, resemble the surface physics of 4D topological crystalline insulators. We establish the above higher-order bulk-boundary correspondence through a dimensional reduction technique, which also allows for a systematic classification of 2D AFHOTIs protected by point group symmetries. We demonstrate applications of our theory to two concrete, experimentally feasible models of AFHOTIs protected by C2 and D4 symmetries, respectively. Our work paves the way for a unified theory for classifying and characterizing Floquet topological matters. *Detailed information about the seminar refer to the email.

会場: via Zoom

イベント公式言語: 英語

Loewner's theorem for maps on operator domains / The structure of maps on the space of all quantum pure states that preserve a fixed quantum angle

2021年5月24日(月) 16:00 - 18:10

森 迪也 (理化学研究所 数理創造プログラム (iTHEMS) 基礎科学特別研究員)

This talk is divided into two independent topics. In the first part of my talk we consider the order structure of hermitian matrices. Given two matrix domains (open connected sets of n-by-n hermitian matrices), what is the general form of order isomorphisms between them? I will explain that there is a complete correspondence between the class of order isomorphisms and that of biholomorphic mappings. In the second part we consider the metric structure of the space P(H) of all quantum pure states (= the projective space of a complex Hilbert space H). Wigner's theorem asserts that every surjective isometry of P(H) onto itself is implemented by a unitary or an antiunitary operator. Uhlhorn generalized Wigner's theorem by showing that every bijective transformation of P(H) that preserves orthogonality is implemented by a unitary or an antiunitary operator. We consider some variants of Uhlhorn's result. The first part is joint work with P. Semrl (Univ. of Ljubljana), and the second part with G.P. Geher (Univ. of Reading). Only basic linear algebra is assumed in both parts.

会場: via Zoom

イベント公式言語: 英語

Aperiodic and amorphous topological phases

2021年5月12日(水) 17:00 - 18:15

クリストファー・ボーン (理化学研究所 数理創造プログラム (iTHEMS) 客員研究員 / 東北大学 材料科学高等研究所 (AIMR) 助教)

Key features of topological insulators and superconductors such as stable edge modes have been found in an increasingly broad class of materials and systems. In this talk, I will introduce a mathematical framework to study Hamiltonians and topological phases on a general class of (aperiodic/random) point atterns. Using techniques from noncommutative geometry, we then show how bulk topological invariants and the bulk-boundary correspondence can be rigorously established in such generic systems. This is based on joint work with Emil Prodan and Bram Mesland. *Detailed information about the seminar refer to the email.

会場: via Zoom

イベント公式言語: 英語

Thermodynamic Uncertainty Relation Connects Physics, Information Science, and Biology

2021年4月28日(水) 13:30 - 16:00

長谷川 禎彦 (東京大学 大学院情報理工学系研究科 准教授)

Higher precision demands more resources. Although this fact is widely accepted, it has only recently been theoretically proved. The thermodynamic uncertainty relation serves as a theoretical basis for this notion, and it states that current fluctuations are bounded from below by thermodynamic costs, such as entropy production and dynamical activity. In this seminar, I show a strong connection between the thermodynamic uncertainty relation and information theory by deriving it through information inequality known as a Cramér-Rao bound, which provides the error bound for any statistical estimator. Moreover, by using a quantum Cramér-Rao bound, I derive a quantum extension of thermodynamic uncertainty relation, which holds for general open quantum systems. The thermodynamic uncertainty relation predicts the fundamental limit of biomolecular processes, and thus it can be applied to infer the entropy production, corresponding to the consumption of adenosine triphosphate, of biological systems in the absence of detailed knowledge about them. *Detailed information about the seminar refer to the email.

会場: via Zoom

イベント公式言語: 英語

Unconventional Spin Transport in Quantum Materials

2021年4月21日(水) 17:00 - 18:15

Se Kwon Kim (Korea Advanced Institute of Science and Technology (KAIST), Republic of Korea)

Recent advancements in spintronic techniques originally developed for spin-based devices now enable us to study fundamental spin physics of various quantum materials with unprecedented spin-current control and measurement, opening a new area of theoretical and experimental investigation of quantum systems. In this talk, we will introduce this emerging research area of spin transport in quantum materials which is fueled by the global interest in quantum information science. As examples, we will discuss our discovery of magnonic topological insulators realized by 2D magnets [1-3], which shows how spintronic techniques can be used for probing elusive quantum materials, and our prediction of long-range spin transport mediated by a vortex liquid in superconductors [4], which shows that quantum materials can provide novel platforms for efficient spin-transport devices. We will conclude the talk by offering a future outlook on quantum spintronics. *Detailed information about the seminar refer to the email.

会場: via Zoom

イベント公式言語: 英語

What "Holography" is and how to use it

2021年4月14日(水) 17:00 - 18:15

Mario Flory (Instituto de Fisica Teorica, Universidad Autonoma de Madrid, Spain)

In this talk, I will give an introduction to the holographic principle and the Anti-de Sitter/Conformal Field Theory (AdS/CFT) correspondence. I will also discuss the role that quantum entanglement plays in this correspondence via the Ryu-Takayanagi formula which maps the calculation of entanglement entropy to a geometric problem of extremal surfaces. Then, I will present a holographic model of a Kondo like effect as an example of how the AdS/CFT correspondence can be employed in practice. *Detailed information about the seminar refer to the email.

会場: via Zoom

イベント公式言語: 英語

Journal Club: Trace inequalities and their applications

2021年4月14日(水) 14:30 - 15:30

後藤 ゆきみ (理化学研究所 数理創造プログラム (iTHEMS) 基礎科学特別研究員)

In this talk, I will explain trace inequalities and related topics. Mainly, I focus on results concerning quantum entropy. This talk is an elementary introduction to that subjects. *Detailed information about the seminar refer to the email.

会場: via Zoom

イベント公式言語: 英語