iTHEMS Seminar

Topological physics and its interdisciplinary influence

November 12 (Wed) 13:00 - 14:00, 2025

Tomoki Ozawa (Professor, Advanced Institute for Materials Research (AIMR), Tohoku University)

Topological insulators are materials which do not conduct current inside but do conduct at the surface or the edge. The name "topological" comes from the fact that the "shape" of the wavefunction of electrons in topological insulators show non-trivial twist, which can be mathematically characterized by the language of topology. Alongside the development of the study of topological insulators in solids, analogous phenomena were found to exist also in other systems such as photonics, mechanics, geophysics, and active matter. In this seminar, I discuss how the underlying concept of "topology of states" can have a broad impact applicable to various areas in physics, with some emphasis on my own contribution to the field. I aim to structure the first half of my seminar to be accessible to those outside physics, and latter half to be more specialized, covering cutting-edge results.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

A Fresh Look at Late-Time Hawking Radiation

September 2 (Tue) 14:00 - 15:30, 2025

Wei-Hsiang Shao (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

There is now a common belief that non-perturbative quantum gravity effects are relevant for resolving the black hole information puzzle. But could such effects also largely alter Hawking radiation itself, the main culprit that led to the puzzle in the first place? There are two main lessons that I would like to convey from this presentation: 1. For large black holes formed by dynamical collapse, the usual description of Hawking radiation in the low-energy effective theory breaks down at an early stage, signaling the need for a UV theory to describe the origin of late-time radiation. 2. In UV models of the radiation field that incorporate a form of nonlocality motivated by string theory, Hawking radiation becomes a transient phenomenon that occurs only for a brief period of time. This behavior suggests a major deviation from the conventional picture of black hole evaporation based on local quantum field theory.

Venue: via Zoom / #359, 3F, RIKEN Wako Campus

Event Official Language: English

Quantum Decryption from technological perspective to business opportunities

June 6 (Fri) 16:00 - 17:30, 2025

Sho Sugiura (CEO, BlocQ, Inc.)

Quantum decryption is a foundational application of fault-tolerant quantum computing (FTQC), essential for future cryptographic security. While quantum simulations, especially quantum chemistry, dominate current quantum computing research, quantum decryption remains less explored despite its significance. In this talk, we give a business style talk that overviews the current status of quantum decryption and our company's pioneering efforts to advance practical FTQC-based solutions. We discuss recent technological advancements and outline our strategic initiatives aimed at leading the field toward secure communications.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

Topology and Brain Science

May 16 (Fri) 14:00 - 15:30, 2025

Shiu Gary (Professor, Department of Physics, University of Wisconsin-Madison, USA)

Venue: via Zoom

Event Official Language: English

Artificial Intelligence and Neuroscience

April 11 (Fri) 14:00 - 15:30, 2025

Junichi Chikazoe (Professor, Center for Brain,Mind and KANSEI Sciences Research, Hiroshima University)

Recent advancements in artificial intelligence have led to various discoveries in the field of neuroscience. For example, it has been demonstrated that the information on orientation columns in the visual cortex and the basic taste information in the gustatory cortex can be extracted by applying machine learning to relatively low-resolution functional MRI data. Additionally, intriguing findings have emerged, such as the information processing structures of artificial neural circuits—designed independently of the brain—showing similarities to those of biological neural networks. In this talk, I will discuss the applications of artificial intelligence in neuroscience and explore future directions in this field.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Omega Meson from Lattice QCD

April 2 (Wed) 15:00 - 16:00, 2025

Haobo Yan (Ph.D. Student, School of Physics, Peking University, China)

The three-body problem, renowned for its unsolvable nature in celestial mechanics and homonymous science fiction, is not only solvable in the quantum realm regarding spectra but also offers profound insights into QCD. In this talk, I will present the first-ever lattice calculation of the resonance parameters for the lightest hadron decaying into three particles, the -meson. By mapping finite-volume energy levels to infinite-volume scattering amplitude, a pole position trajectory is obtained that, when extrapolated to the physical point, shows good agreement with the experiment.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

The puzzle of angular momentum conservation in beta decay and related processes.

March 21 (Fri) 14:00 - 15:30, 2025

Gordon Baym (Professor Emeritus, University of Illinois, USA)

This is a iTHEMS-FQSP joint seminar. We ask the question of how angular momentum is conserved in a number of related processes, from elastic scattering of a circularly polarized photon by an atom, where the scattered photon has a different spin direction than the original photon; to scattering of a fully relativistic spin-1/2 particle by a central potential; to inverse beta decay in which an electron is emitted following the capture of a neutrino on a nucleus, where the final spin is in a different direction than that of the neutrino – an apparent change of angular momentum. The apparent non-conservation of angular momentum arises in the quantum measurement process in which the measuring apparatus does not have an initially well-defined angular momentum, but is localized in direction in the outside world. We generalize the discussion to massive neutrinos and electrons, and examine nuclear beta decay and electron-positron annihilation processes through the same lens, enabling physically transparent derivations of angular and helicity distributions in these reactions.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

The Golden Age of Neutron Stars

March 17 (Mon) 15:30 - 17:00, 2025

Gordon Baym (Professor Emeritus, University of Illinois, USA)

This is a iTHEMS - Nishina Center Joint Seminar. Neutron stars were first posited in the early thirties, and discovered as pulsars in the late sixties; however we are only recently beginning to understand the matter they contain. After touching briefly on the history of neutron stars, I will describe the ongoing development of a consistent picture of the liquid interiors of neutron stars, now driven by ever increasing observations as well as theoretical advances. These include, in particular. observations of at least three heavy neutron stars of about 2.0 solar masses and higher; ongoing simultaneous inferences of masses and radii of neutron stars by the NICER telescope; and past and future observations of binary neutron star mergers, through gravitational waves as well as across the electromagnetic spectrum. I will also discuss pulsar timing arrays to detect very long wavelength gravitational waves, a remarkable role for neutron stars. Theoretically an understanding is emerging in QCD of how nuclear matter can turn into deconfined quark matter in the interior, and be capable of supporting heavy neutron stars, which I will illustrate with a discussion of modern quark-hadron crossover equations of state.

Venue: 2F Large Meeting Room, RIBF Building (E01) (Main Venue) / via Zoom

Event Official Language: English

Applications of Geometry of Numbers to Phyllotaxis and Crystallography

February 28 (Fri) 14:00 - 15:30, 2025

Ryoko Oishi-Tomiyasu (Professor, Institute of Mathematics for Industry, Kyushu University)

The golden angle method, originally known from phyllotaxis in botany, has been used to generate dense point packings on surfaces of revolution. In my recent work, I have extended this method to general surfaces and higher-dimensional manifolds by employing the theories of products of linear forms in number theory, diagonalizable metrics in differential geometry, and local solutions of quasilinear hyperbolic equations. This extension suggests that any biological forms can exhibit phyllotactic patterns locally regardless of their morphology, while the overall pattern is influenced by their global properties in the embedded space. On the algebraic side, it is interesting that the same ideas used for phyllotaxis can also be applied to pseudorandom number generation over F2 = {0, 1}. This work is motivated by my previous research in crystallography. Time permitting, I will also introduce some of the research, which contributes to the analytical foundations of crystallography and is also an application of the geometry of numbers.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Mathematical Studies on Human Cooperation

February 12 (Wed) 15:00 - 17:00, 2025

Yohsuke Murase (Research Scientist, Discrete Event Simulation Research Team, RIKEN Center for Computational Science (R-CCS))

Cooperation is a fundamental part of human society. But from an evolutionary perspective, it remains a puzzle—why do people help others even when it costs them? In theory, selfish individuals should have an advantage over cooperators. To explain how cooperative behaviors evolved, researchers have proposed several mechanisms, among which direct and indirect reciprocity play key roles in human interactions. In this talk, I will present my research on the evolution of cooperation, focusing on these two mechanisms. I will begin with an introduction to game theory and evolutionary game theory, which help us understand how people make decisions in strategic situations. Then, I will discuss my study on the repeated Prisoner’s Dilemma, where we discovered a new class of strategies through mathematical analysis and large-scale computations [1]. Finally, I will talk about my research on indirect reciprocity, a process where people cooperate based on reputation [2].

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Mathematics of the Future, Science of the Future: Large Language Models and Their Applications

February 6 (Thu) 14:00 - 16:00, 2025

Akiyoshi Sannai (Program-Specific Associate Professor, Graduate School of Science, Kyoto University)

In recent years, the rapid development of large language models (LLMs) such as ChatGPT has given many researchers a strong impression that these systems truly exhibit “intelligence.” In this presentation, we first review the evolution of AI research, explaining how large language models go beyond conventional machine learning by enabling more “general” forms of learning. We then highlight the importance of “sensors” and “mathematical capability” as key factors that allow AI to autonomously carry out scientific tasks such as problem analysis, hypothesis generation, and proofs in fields like mathematics and physics. We also examine how proof assistants can address the issue of hallucinations in LLM outputs, and discuss the role of combinatorial creativity in accelerating interdisciplinary research. Finally, we introduce our “AI Mathematician” agent project, demonstrating how integrating large language models with proof assistants can open new horizons in mathematical sciences.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Hopfions in Condensed Matter and Field Theory

December 16 (Mon) 16:00 - 17:30, 2024

Avadh Saxena (Professor, Los Alamos National Laboratory, USA)

Abstract: Nontrivial topological defects such as knotted solitons called hopfions have been observed in a variety of materials including chiral magnets, nematic liquid crystals and even in ferroelectrics as well as studied in other physical contexts such as Bose-Einstein condensates. These topological entities can be modeled using the relevant physical variable, e.g., magnetization, polarization or the director field. Specifically, we find exact static soliton solutions for the unit spin vector field of an inhomogeneous, anisotropic three-dimensional (3D) Heisenberg ferromagnet and calculate the corresponding Hopf invariant H analytically and obtain an integer, demonstrating that these solitons are indeed hopfions [1]. H is a product of two integers, the first being the usual winding number of a skyrmion in two dimensions, while the second encodes the periodicity in the third dimension. We also study the underlying geometry of H, by mapping the 3D unit vector field to tangent vectors of three appropriately defined space curves. Our analysis shows that a certain intrinsic twist is necessary to yield a nontrivial topological invariant: linking number [2]. Finally, we focus on the formation energy of hopfions to study their properties for potential applications. Short bio: Avadh Saxena is former Group Leader of the Condensed Matter and Complex Systems group (T-4) at Los Alamos National Lab, New Mexico, USA where he has been since 1990. He is also an affiliate of the Center for Nonlinear Studies at Los Alamos. His main research interests include phase transitions, optical, electronic, vibrational, transport and magnetic properties of functional materials, device physics, soft condensed matter, non-Hermitian quantum mechanics, geometry, topology and nonlinear phenomena & materials harboring topological defects such as solitons, polarons, excitons, breathers, skyrmions and hopfions. He recently completed a book on “Phase Transitions from a Materials Perspective” (Cambridge University Press, 2024). He is an Affiliate Professor at the Royal Institute of Technology (KTH), Stockholm, Sweden and holds adjunct professor positions at the University of Barcelona, Spain, University of Crete, Greece, Virginia Tech and the University of Arizona, Tucson. He is Scientific Advisor to National Institute for Materials Science (NIMS), Tsukuba, Japan. He is a Fellow of Los Alamos National Lab, a Fellow of the American Physical Society (APS), a Fellow of the Japan Society for the Promotion of Science (JSPS) and a member of the Sigma Xi Scientific Research Society, APS and American Ceramic Society (ACerS).

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Studying quark-gluon plasma with multi-stage dynamical models in relativistic nuclear collisions

December 10 (Tue) 15:30 - 17:00, 2024

Yuuka Kanakubo (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

A collision of relativistically accelerated large nuclei creates the hottest matter on Earth — quark-gluon plasma (QGP). The properties of QGP have been studied through comparisons of final-state particle distributions between theoretical models and experimental data. To quantitatively constrain QGP properties, it is necessary to build Monte Carlo models that simulate the space-time evolution of the system throughout the entire collision process. This includes the initial matter production from the accelerated nuclei, the evolution of QGP, hadronisation, and the evolution of hadron gas. In this talk, I will first explain how theoretical models, based on relativistic hydrodynamics and hadronic transport, are conventionally built and how they successfully extract QGP properties. Next, I will discuss a hot topic: the possibility of finding QGP in proton-proton collisions, based on results from a state-of-the-art model that includes both equilibrated and non-equilibrated systems. Also, I will introduce a novel Monte Carlo initial state model based on perturbative QCD minijet production supplemented with a saturation picture. This Monte-Carlo EKRT model is one of the first initial state models for hydrodynamics to describe initial particle production from small to large momentum within a single framework, where total energy-momentum and charge conservations are imposed.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Entanglement of astrophysical neutrinos

December 10 (Tue) 13:30 - 15:00, 2024

Baha Balantekin (Eugene P. Wigner Professor, Department of Physics, University of Wisconsin-Madison, USA)

Collective oscillations of neutrinos represent emergent nonlinear flavor evolution phenomena instigated by neutrino-neutrino interactions in astrophysical environments with sufficiently high neutrino densities. In this talk, after a brief introduction, it will be shown that neutrinos exhibit interesting entanglement behavior in simplified models of those oscillations. Attempts to study this behavior using classical and quantum computers will be described. An intriguing connection to the heavy-element nucleosynthesis, namely the possibility of neutrino entanglement driving a new kind of i-process nucleosynthesis, will be introduced,

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Crop domestication

November 25 (Mon) 15:00 - 17:00, 2024

Cheng-Ruei Lee (Professor, Institute of Ecology and Evolutionary Biology, National Taiwan University, Taiwan)
Jeffrey Fawcett (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

This is a joint seminar hosted by the Mathematical Biology lab of Kyushu University where Jeffrey Fawcett (iTHEMS) and Cheng-Ruei Lee (National Taiwan University) will give talks about plant domestication. Both talks will be aimed at students and will include some basic introduction of the topic. The seminar will be held on-site at Kyushu University and also by zoom so please free to register and join. Program: Title: Domestication and dispersal process of common buckwheat Speaker: Dr. Jeffrey Fawcett (RIKEN iTHEMS) Abstract: Crop domestication has not only been an ideal model to study how selection drives evolution, it is also tightly linked to past human activity and contains useful information that can improve plant breeding. Common buckwheat (Fagopyrum esculentum), which is used to make “soba” noodles in Japan, was domesticated from a wild progenitor species distributed in Southwest China. We have been using whole-genome sequences of several hundred cultivated accessions from around the world and some wild progenitor accessions to study its process of domestication and subsequent dispersal throughout Eurasia including Japan. In this talk, I will first provide an overview of the domestication and dispersal process of common buckwheat based on archaeological findings. I will then discuss the domestication and dispersal process and adaptive evolution of common buckwheat based on results of our population genetic analyses [1]. Title: The domestication and expansion history of mung bean and adzuki bean: evidence from population genomics Speaker: Prof. Cheng-Ruei Lee (National Taiwan University) Abstract: Who domesticated the crops we eat? When and where? What happened after domestication? How did crops spread across the world? These are the questions that have fascinated archaeologists for a long time. Using modern genomics techniques, we aim to answer these questions from a different angle. In mung bean (Vigna radiata), we uncovered a unique route of post-domestication range expansion. This route cannot be explained simply by human activities alone; instead, it is highly associated with the natural climates across Asia. We showed how the trans-continental climatic variability affected the range expansion of a crop and further influenced local agricultural practices and the agronomic properties of mung bean varieties. In adzuki (Vigna angularis), we obtained solid evidence of its domestication in Japan, most likely by the Jomons. We identified and validated the causal mutations for the seed coat color change during domestication. Contrary to the common belief that important yield-ensuring phenotypes (e.g., loss of pod shattering) should be selected early during domestication, we revealed a unique order of domestication trait evolution that cannot be observed from archaeological records directly [2]. Please register via the form by November 22nd (Fri.). We will share the Zoom link with online participants on the morning of the event day.

Venue: W1-C-909, Ito Campus, Kyushu University /

Event Official Language: English

Emergence of wormholes from quantum chaos

November 12 (Tue) 16:30 - 18:00, 2024

Gabriele Di Ubaldo (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

I will give a broad introduction to some aspects of quantum gravity and the so-called black hole information problem. I will introduce wormholes as novel contributions to the gravitational path integral and how they provide a solution to the black hole information problem. Wormholes, however, are rather mysterious and we don’t have a good microscopic understanding of them and why we should include them in the our theory. In particular, wormholes seem to imply that gravity is not a proper quantum system but rather an average over a statistical ensemble of quantum systems. I will then transition into my own work which addresses these questions in the context of holography. I will show how wormholes in 3D quantum gravity can emerge from quantum chaos in the dual 2D Conformal Field Theory, without averaging. Wormholes capture coarse-grained properties of the CFT and conversely an individual chaotic CFT can effectively behave as an averaged system. Furthermore we will be able to explicitly factorize wormholes to extract microscopic information on black hole microstates. To achieve this I will (briefly) introduce and use tools such as Random Matrix Theory, the Gutzwiller Trace formula and Berry’s diagonal approximation, and the theory of SL(2,Z) non-holomorphic modular forms.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Dynamics of Phase Transitions: Between First and Second Order

October 8 (Tue) 16:00 - 17:30, 2024

Fumika Suzuki (CNLS Postdoctoral Research Associate, T4 / Center for Nonlinear Studies, Los Alamos National Laboratory, USA)

Phase transitions are typically classified as either first-order or second-order. The formation of topological defects in second-order phase transitions is well described by the Kibble-Zurek mechanism, while nucleation theory addresses first-order phase transitions. However, certain systems, such as superconductors and liquid crystals, can exhibit “weakly first-order” phase transitions that do not fit into these established frameworks. In this presentation, I introduce a new theoretical approach that combines the Kibble-Zurek mechanism with nucleation theory to explain topological defect formation in weakly first-order phase transitions. Additionally, I will discuss nonlinear quantum phase transitions that exhibit behaviors similar to weakly first-order transitions, which can be related to experiments with ultra-cold Rydberg atoms.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Stringy Nonlocality: Operator Formalism and Implications

July 26 (Fri) 14:00 - 15:30, 2024

Wei-Hsiang Shao (Ph.D. Student, Department of Physics, National Taiwan University, Taiwan)

Nonlocality is a fundamental property of string theory, where point-like particles are replaced by extended strings. This feature is especially evident in string field theories, where field components interact through form factors containing spacetime derivatives of infinite order. The usual approach to canonical quantization is no longer applicable, and thus a non-perturbative treatment of nonlocal effects at the quantum level remains unclear. In this seminar, I will discuss a recent attempt to construct an operator formalism for stringy nonlocal field theories, and explore the potential implications for black hole radiation and primordial fluctuations in the early universe.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English