iTHEMS Seminar

Scattering amplitude from quantum computing with reduction formula

August 27 (Thu) 15:00 - 16:00, 2026

Wai Kin Lai (Lecturer, The University of Hong Kong, China)

In this talk, I will present a new general framework for computing scattering amplitudes in quantum field theory with quantum computers in a fully nonperturbative way. After giving a motivation for the application of quantum computing to particle physics, I will briefly introduce the basics of quantum computing. After that, I will give the details of the proposed quantum algorithm for calculating scattering amplitudes, which utilizes the Lehmann-Symanzik-Zimmermann (LSZ) reduction formula. In this framework, one only has to construct one-particle states of zero momentum, and no wave packets of incoming particles are needed. The framework is able to incorporate scatterings of bound states, and is ideal for scatterings involving a small number of particles. This framework is expected to have particular advantages when applied to exclusive hadron scatterings. As a proof of concept, by simulations on classical hardware, I demonstrate that in the one-flavor Gross-Neveu model, the fermion propagator, the connected fermion four-point function, and the propagator of a fermion-antifermion bound state obtained from our proposed quantum algorithm have the desired pole structure crucial to the implementation of the LSZ reduction formula. Brief CV: Wai Kin Lai received PhD in physics at the University of Pittsburgh in 2016. He was a postdoctoral reseacher at the Technical University of Munich from 2016 to 2020. From 2020 to 2023, he was a postdoctoral researcher at South China Normal University, and was a visiting assistant researcher at the University of California, Los Angeles from 2022 to 2023. Wai Kin Lai is currently a lecturer at HKU SPACE in Hong Kong. Wai Kin Lai's research interests are applications of effective field theories of QCD to particle phenomenology and studies of nonperturbative physics with quantum computation.

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

Event Official Language: English

Current challenges teaching undergraduate first year physics in Canada

July 30 (Thu) 13:00 - 14:00, 2026

Catherine Beauchemin (Deputy Director, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

I will talk about the challenges of teaching first year physics in 2026 to undergraduate students in computer science and engineering in Canada. Topics will include issues with textbook publishers (online vs physical books), open access textbook/homework systems, teaching, learning and setting evaluations, labs and homework in the era of AI, student attitudes towards learning, academic accommodation for disabilities, etc. This seminar could be interesting to those of you who will face teaching in your future, especially abroad. Although my perspective is based on physics and Canada, a number of issues raised are broadly relevant to other fields and countries.

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

Event Official Language: English

When Is Collective Intelligence a Lottery? Toward a Physics of Bounded Agent Societies

July 22 (Wed) 16:00 - 17:30, 2026

Hidenori Tanaka (Group Leader, CBS-NTT Physics of Intelligence Program, Center for Brain Science, Harvard University, USA)

Multi-agent LLM systems can reach agreement quickly, but agreement alone does not reveal whether a group has integrated evidence, amplified a bias, or merely locked in the luck of early samples. In this talk, I will develop a physics-style approach to this problem through two synthetic games. In reward-free naming games, populations form conventions through mutual in-context learning: agents treat one another's sampled outputs as evidence, so early fluctuations compound into consensus. A minimal model, Quantized Simplex Gossip (QSG), identifies this regime as memetic drift and predicts scaling laws and a crossover from lottery-like drift to bias-driven selection. I will then introduce the Flag Game, in which a hidden country flag provides verifiable ground truth but each agent sees only a private crop of it. Here, bounded agents must balance private evidence against social input. We find rich phenomenologies, where adding agents can help or hurt, large groups can polarize, and social-awareness prompting, model diversity, and organizational structure all reshape collective performance. Extending QSG with grounded evidence and model-specific update rules explains these effects. More broadly, these studies frame an LLM society as a network of networks, neural networks coupled through social interaction, and outline a route toward a multiscale physics of interacting AI agents, linking model-internal representations, agent-level decision mechanisms, and population-level social dynamics.

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

Event Official Language: English

iTHEMS-FQSP joint seminar “Aspects of Tripartite Haar Random States”

July 8 (Wed) 10:30 - 12:00, 2026

Beni Yoshida (Research Faculty Senior Faculty, Perimeter Institute for Theoretical Physics, Canada / Senior Visiting Scientist, Fundamental Quantum Science Program, RIKEN)

Randomness plays central roles in understanding strongly entangled quantum systems. The foundational result is Page’s theorem: a bipartite Haar random state is nearly maximally entangled. In this talk, I will ask what happens when the system is divided into three parts. We show that tripartite Haar random states have a very different structure: when each subsystem contains fewer than half of the total qubits, no EPR-like bipartite entanglement can be distilled between any pair by local unitaries or local operations. I will discuss several consequences of this observation, including its implications for quantum error correction, complementary recovery, connected entanglement wedges in AdS/CFT, and possible baby-universe degrees of freedom.

Venue: Okochi Hall (Main Venue) / via Zoom

Event Official Language: English

Fermionic modes of D-instanton wormholes from broken local supersymmetry

June 24 (Wed) 15:30 - 17:00, 2026

Hiroshi Itoyama (Specially Appointed Professor, Nambu Yoichiro Institute of Theoretical and Experimental Physics (NITEP), Osaka Metropolitan University)

In low-energy supergravity treatment of type IIB superstring on general D-instanton wormhole profiles in the bulk, we obtain non-vanishing scalar two-point functions in addition to the vanishing　〈τ＊τ＊〉 that corresponds to the BPS amplitude detected by two D-instantons at their respective boundaries. This is exploited to show that the modes of broken local supersymmetry in the bulk deliver the fermionic (diagonal) modes on the boundaries through the deformation by the form of current-current two point functions propagating on the tree level cylinder geometry. Our treatment is generalizable to multi D-instanton cases and general Euclidean branes.

Venue: #359, 3F, Main Research Building (Main Venue) / via Zoom

Event Official Language: English

Disorder and Defects in Critical Systems

June 8 (Mon) 13:30 - 15:00, 2026

Baishali Roy (Postdoctoral Fellow, Indian Institute of Technology Kanpur, India)

Real critical systems are often constrained by boundaries and affected by impurities. In 3d, the effect of disordered impurities on the boundary can be modeled by a random magnetic field on a two-dimensional defect. In this talk, I will discuss how such disorder affects the Wilson-Fisher fixed point in d=4−\epsilon dimensions. By analyzing the one-loop RG flow of the defect couplings using the replica formalism, we find a non-trivial "dirty" fixed point which represents a new boundary universality class, stabilized by the bulk \phi^4 interaction. Disordered systems at critical points are known to exhibit logarithmic behavior — I will also discuss how operator mixing in the replica limit gives rise to a logarithmic defect CFT in our setup.

Venue: #359, 3F, Main Research Building (Main Venue) / via Zoom

Event Official Language: English

Closed Seminar on Quantum Topology and Related Topics

May 29 (Fri) 14:00 - 18:00, 2026

Mao Hoshino (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Kan Kitamura (Assistant Professor, Department of Mathematics, College of Science, Rikkyo University)
Yuya Murakami (Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Vladimir Sosnilo (Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

We will hold a closed seminar on quantum topology and related topics. The talks will be given by the following four speakers. The talks will not be streamed online or recorded. 14:00–14:30 Mao Hoshino 14:30–15:00 Kan Kitamura (15:00–15:30 Coffee break) 15:30–16:00 Yuya Murakami 16:00–16:30 Vladimir Sosnilo (16:30–17:30 Casual reception)

Venue: Seminar Room #359

Event Official Language: English

Seeing Photons, from Einstein to Bohr to Hanbury Brown-Twiss and related Hong-Ou-Mandel Interference Phenomena

April 22 (Wed) 15:00 - 16:30, 2026

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

Why do we believe that the electromagnetic field is quantized, and photons exist? This talk will focus on two ways that the quantization of the electromagnetic field manifests itself in interference experiments. Bohr, who initially doubted photons after Einstein's initial proposal of the photon to explain the photoeffect, eventually proposed a thought experiment showing that the consistency of elementary quantum mechanics at the level of two slit diffraction requires the quantization of the electromagnetic field. In addition, as I will argue, both Hanbury Brown-Twiss interferometry and the closely related Hong-Ou-Mandel effect provide yet another way to see that the electromagnetic field must be quantized.

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

Event Official Language: English

RIKEN Seminar: Formulation of Life Phenomena from Quantum Theory

April 16 (Thu) 14:00 - 16:05, 2026

13:45 Opening 14:00-14:05 Introduction Atsushi Iriki (Teikyo University Advanced Comprehensive Research Organization Division of Artificial Intelligence, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS)) 14:05-14:35 "Interpretation of Life Phenomena Using Quantum Wave Functions and Field Theory" Kazuhiro Sakurada (Keio University Medical School and RIKEN Center for Integrative Medical Sciences (IMS), Predictive Medicine Special Project (PMSP)) 14:35-14:45 Q&A 14:45-15:30 "Bridging neurophysiology and quantum-like cognition" Andrei Khrennikov (Center for Mathematical Modeling in Physics and Cognitive Sciences Linnaeus University) 15:30-15:45 Q&A 15:45-16:00 "Quantum-Like Measurement" Masanao Ozawa (RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS), RIKEN TRIP FQSP, and Nagoya University) 16:00-16:05 Closing Remarks Satoshi Iso (Director, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS)) Host Laboratory: Predictive Medicine Special Project, RIKEN Center for Integrative Medical Sciences (IMS) / RIKEN Center for Interdisciplinary Theoretical and Mathematical Science (iTHEMS) *Registration is required by April 14 via the registration form. Contact: Predictive Medicine Special Project (pmsp-web@ml.riken.jp)

Venue: Meeting Room 305, Brain Science Ikenohata Research Bldg. (C56), RIKEN Wako Campus

Event Official Language: English

The career talk: From Quarks to Cinematic Sparks

February 27 (Fri) 15:00 - 16:30, 2026

Agnes Mocsy (Professor, Department of Mathematics and Science, Pratt Institute, USA)

While my career began in a linear way, it gradually opened into a non-traditional path through unexpected mergings, where theoretical nuclear physics, filmmaking, and creative public and academic engagement intertwined. I will share how scientific inquiry, artistic practice, and storytelling began shaping one another, opening new ways to explore complexity, emotion, and connection. Drawing on work from my physics research to cinema projects like Rare Connections, I will reflect on how curiosity and creative thinking move freely across science and art, deepening each and expanding how we understand the human experience. My aim is to offer a perspective on the possibilities that emerge when we allow our multitudes to meet and transform one another.

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

Event Official Language: English

Higher Gauge Structures and Invariant Action Principles

January 6 (Tue) 15:15 - 16:15, 2026

Sebastián Salgado (External Researcher, Instituto de Alta Investigacion, Universidad de Tarapaca, Chile)

I present the systematic construction of gauge theories based on free differential and L-infinity algebras. This provides a consistent algebraic framework for constructing gauge-invariant theories whose field content is extended by higher-degree differential forms as gauge potentials. I derive explicit expressions for the corresponding extended Chern-Simons actions and the generalized anomaly terms that emerge from them. Possible applications to gravity and supergravity will also be discussed.

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

Event Official Language: English

Invitation to Random Tensor Models: from random geometry, enumeration of tensor invariants, to characteristic polynomials

January 6 (Tue) 13:30 - 14:30, 2026

Reiko Toriumi (Associate Professor, Okinawa Institute of Science and Technology Graduate University (OIST))

I will introduce random tensor models by first reviewing their motivation coming from random geometric approach to quantum gravity. Then, I will selectively present some of the interesting research results, by highlighting recent results on enumeration of graphs representing tensor invariants, and reporting our recent work on a new notion of characteristic polynomials for tensors via Grassmann integrals and distributions of roots of random tensors. The latter two are based on arXiv:2404.16404[hep-th] and arXiv:2510.04068[math-ph]

Venue: #359, 3F, Main Research Building (Main Venue) / via Zoom

Event Official Language: English

About analytic continuation of quantum field theories in non-integer dimensions

December 5 (Fri) 14:00 - 15:30, 2025

Slava Rychkov (Professor, Institut des hautes études scientifiques, France)

Analytic continuation in dimension has been used first as a way to regularize perturbative quantum field theory. But since the work of Wilson and Fisher, quantum field theory in d-dimension has been used more radically, to connect theories living say, in d=4, to theories in d=3 and d=2. Mathematically it's not fully clear what this means. I will give some thoughts about this subject, and I will describe some recent paradoxes which arise when one consider expansion of O(N) models around d=2, based on recent work with Fabiana De Cesare.

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

Event Official Language: English

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

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