Quantum States Over Time: From Foundations To Applications

March 24 (Tue) 15:30 - 17:00, 2026

Minjeong Song (Research Fellow, Centre for Quantum Technologies, National University of Singapore, Singapore)

In this talk, I will introduce quantum states over time (QSOT), a formalism for describing quantum systems over space-time. I will begin by reviewing how QSOT has emerged in the literature. While conventional density operator formalism has been effective across many areas of quantum information theory, QSOT was developed to meet more specialized research needs— most notably, as a key ingredient to develop a quantum version of Bayes’ theorem. I will end the first part of my talk by comparing various QSOT that have been proposed. In the second part, I will discuss the causal compatibility problem as an application of QSOT. I will focus on the temporal compatibility problem, which asks the following: from correlations in measurement outcomes alone, can two otherwise isolated parties establish whether such correlations are atemporal (i.e., temporally incompatible)? That is, can they rule out that they have been given the same system at two different times? I will first explain how characterizing measurement statistics in a causal agnostic scenario is equivalent to characterizing a specific type of QSOT, known as pseudo-density operators. I will then present our recent findings obtained by analyzing pseudo-density operators; In particular, we demonstrate that atemporality is distinct from entanglement, though they appear to be equivalent at first glance. Specifically, we show atemporality implies entanglement, but not vice versa, thus revealing that atemporality is a strictly stronger form of quantum correlations than entanglement. Nevertheless, we also find that sufficiently strong entanglement does imply atemporality.

Venue: #359, Seminar Room #359

Event Official Language: English

Quantum Simulation of Non-Abelian Gauge Theories: Correcting Common Misconceptions (1/3)

March 24 (Tue) 18:00 - 19:00, 2026

Masanori Hanada (Reader, School of Mathematical Sciences, Queen Mary University of London, UK)

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

Event Official Language: Japanese

Toward a Mathematical Prehistory of Homo sapiens: Data Integration and Statistical Representation in PaleoAsiaDB

March 26 (Thu) 13:00 - 14:00, 2026

Kenji Okubo (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

This talk introduces PaleoAsiaDB, a curated database of lithic assemblages from Paleolithic Asia, and aims to initiate a discussion on its potential uses and methodological challenges. The database integrates information on tool typology, technological attributes, stratigraphy, and chronological ranges across multiple sites and periods. Archaeological assemblage data are inherently heterogeneous, combining categorical variables with hierarchical structure and, in some cases, continuous measurements. In addition, temporal information is often represented as ranges rather than precise dates, and sampling intensity varies substantially across sites. These features make it non-trivial to define consistent procedures for comparison, aggregation, and quantitative analysis. The goal of this session is to gather feedback on data representation and analysis strategies, and to clarify what types of quantitative approaches are most suitable for extracting robust patterns from archaeological assemblage data.

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

Event Official Language: English

Application of a one-dimensional scheme to model diurnal water temperature fluctuations near the surface of a stratified lake

March 27 (Fri) 10:30 - 12:00, 2026

John Craig Wells (Professor, College of Science and Engineering Department of Civil and Environmental Engineering, Ritsumeikan University / Senior Visiting Scientist, Data Assimilation Research Team, RIKEN Center for Computational Science (R-CCS))

When simulating the atmosphere across various scales, accurately resolving the diurnal warming of sea and lake surfaces is a critical requirement. For example, regional atmospheric models must correctly simulate air-water temperature gradients to successfully capture mesoscale circulations such as sea and lake breezes. Often the SST (or Lake Surface Temperature LST) applied to the atmospheric simulator is modelled using a “slab model” of a certain thickness and thermal mass. However slab models often predict diurnal variation of SST poorly. In this talk I will discuss preliminary results from “DiuSST”, recently proposed by R. Börner et al (2025; https://doi.org/10.5194/gmd-18-1333-2025) to provide boundary conditions for diurnally varying SST to atmospheric simulators. Börner et al ’s testing and validation of DiuSST was based on an ocean cruise that measured skin surface temperature with an infrared radiometer, and water temperature at 3m depth. By contrast I cross-check DiuSST results against near-surface temperature profiles in a stratified lake, Lake Biwa, that were recorded at nearshore and offshore locations during the early summer of 2021.

Venue: Hybrid Format (RIKEN R-CCS room 107 and Zoom)

Event Official Language: English

QFT as a set of ODEs

March 27 (Fri) 13:30 - 15:30, 2026

Qiao Jiaxin (Project Researcher, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)

Correlation functions of local operators in Quantum Field Theory (QFT) on hyperbolic space can be fully characterized by the set of QFT data. These are the scaling dimensions of boundary operators, the boundary Operator Product Expansion (OPE) coefficients and the Boundary Operator Expansion (BOE) coefficients that characterize how each bulk operator can be expanded in terms of boundary operators. For simplicity, we focus on two dimensional QFTs and derive a universal set of first order Ordinary Differential Equations (ODEs) that encode the variation of the QFT data under an infinitesimal change of a bulk relevant coupling. In principle, our ODEs can be used to follow a renormalization group flow starting from a solvable QFT into a strongly coupled phase and to the flat space limit.

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

Event Official Language: English

Quantum Simulation of Non-Abelian Gauge Theories: Correcting Common Misconceptions (2/3)

March 31 (Tue) 18:00 - 19:00, 2026

Masanori Hanada (Reader, School of Mathematical Sciences, Queen Mary University of London, UK)

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

Event Official Language: Japanese

A mathematical promenade in microscopic locomotion

April 2 (Thu) 13:00 - 14:00, 2026

Clément Moreau (CNRS Researcher, CNRS, France)

The microscopic world offers a fascinating diversity of locomotion strategies, relying primarily on the use of flagella and cilia. These slender structures, capable of complex periodic deformations, serve as a major source of inspiration for medical microrobotics. At this scale, fluid dynamics is governed by the predominance of viscosity over inertia. This low-Reynolds number regime imposes strict physical constraints, summarized by the famous « scallop theorem »: a reciprocal deformation cannot produce any net displacement. Mathematically, this is framed by the Stokes connection, which links changes in body shape to net movement in space. This presentation proposes a journey through the modeling principles of microscopic swimmers. We will see how to derive analytical solutions to the locomotion problem by simplifying degrees of freedom or by assuming small deformation amplitudes. I will then present the perspective of control theory to address the « controllability » property, i.e. the ability of a locomotor to reach any target position and shape. Finally, I will question a classic hypothesis in the field: the inextensibility of flagella. Although the literature often assumes these structures are rigid in the longitudinal direction, certain micro-organisms and artificial robots exhibit significant compression variations. I will present recent results, based on classical modeling tools, exploring the influence of compression-curvature coupling on locomotion efficiency at low Reynolds numbers.

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

Event Official Language: English

Tensor networks for QCD in the strong-coupling expansion

April 2 (Thu) 15:30 - 17:00, 2026

Tilo Wettig (Professor, Universität Regensburg, Germany)

We present the order-separated Grassmann higher-order tensor renormalization group (OS- GHOTRG) method for QCD with staggered quarks in the strong-coupling expansion. The method allows us to determine the expansion coefficients of the partition function, from which we can obtain the strong-coupling expansions of thermodynamical observables. We use the method in two dimensions to compute the free energy, the particle-number density, and the chiral condensate as a function of the chemical potential up to third order in the inverse coupling 𝛽. Although the expansion itself is only a good approximation to the full theory at small 𝛽, we show that in the vicinity of the phase transition the range of applicability can be greatly extended by fits to judiciously chosen transition functions. These fits also yield a valuable expansion of the critical chemical potential in 𝛽. https://www.uni-regensburg.de/physics/hep/people/professors/wettig/index.html

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

Event Official Language: English

A Hybrid Pseudo-spectral–PINN Approach to Black Hole Quasinormal Modes

April 3 (Fri) 14:00 - 15:15, 2026

Alexandre M. Pombo (PD, Institute of Physics of the Czech Academy of Sciences, Czechia)

Gravitational-wave detections by the LIGO-Virgo-KAGRA network have turned compact-object mergers into precision probes of strong gravity. The post-merger ringdown is particularly incisive: it is governed by quasinormal modes (QNMs), the damped oscillations that encode the remnant's structure and provide a fingerprint of the final object. While current detectors constrain the dominant mode, next-generation observatories will resolve multiple modes with high precision, placing stringent demands on the accuracy of theoretical predictions. Computing QNMs for rotating black holes is, however, a non-trivial task, as it requires solving highly coupled, complex-valued perturbation equations where standard methods struggle. In this talk, I present SpectralPINN, a hybrid solver combining Pseudo-spectral methods with Physics-Informed Neural Networks, validated at 10⁻⁵ relative accuracy. I will present results for Kerr and Kerr-Newman black holes, demonstrating the method's robustness and accuracy across parameter space, and discuss its potential for extension to more exotic compact objects relevant to next-generation detector science.

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

Event Official Language: English

Quantum Simulation of Non-Abelian Gauge Theories: Correcting Common Misconceptions (3/3)

April 7 (Tue) 18:00 - 19:30, 2026

Masanori Hanada (Reader, School of Mathematical Sciences, Queen Mary University of London, UK)

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

Event Official Language: Japanese

Biology Starter Meeting & Welcome 4 New Members!

April 9 (Thu) 13:00 - 15:00, 2026

Alba Nieto Heredia (Postdoctoral Researcher, Mathematical Genomics RIKEN ECL Research Unit, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Takehiro Tottori (Special Postdoctoral Researcher, RIKEN Center for Brain Science (CBS))
Mariia Ivonina (Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Satsuki Hirasawa (Ph.D. Student, Graduate School of Information Science and Technology, Hokkaido University)

This is a special 2 h event of our newly renewed Biology Study Group! This year, 4 new members are joining iTHEMS Biology. They will each give us a 15 min introduction to their research. All participants will also take 2-3 min to introduce themselves and their research topic to the new members. If time permits, we'll hold a brief organizational meeting to review the running of the biology seminars in the new fiscal year. We strongly encourage all iTHEMS members, not just biology-interested ones, to join our session at least in the 1st hour, to meet the new members and learn about their research.

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

Event Official Language: English

Clumpy Outflows from Super-Eddington Accreting Black Holes

April 10 (Fri) 14:00 - 15:15, 2026

Haojie Hu (JSPS Research Fellow, University of Tsukuba)

Recent advances in X-ray spectroscopic observation have enabled researchers to reveal distinct clumpy structures in the super-Eddington outflows from the supermassive black hole in PDS 456 (XRISM Collaboration 2025), initiating detailed investigation of fine-scale structures in accretion-driven outflows. In this talk, I will introduce our high-resolution, two-dimensional radiation-hydrodynamics simulations with time-varying and anisotropic initial and boundary conditions that reproduce clumpy outflows from super-Eddington accretion flows. The resulting clumpy outflows extend across a wide range of radial distances and polar angles, exhibiting typical properties such as a size of ~10 rg (where rg is the gravitational radius), a velocity of ~0.05–0.2 c (where c is the speed of light), and about five clumps along the line of sight. Although the velocities are slightly smaller, these characteristics reasonably resemble those obtained from the XRISM observation. The gas density of the clumps is on the order of 10^-13–10^-12 g cm^-3, and their optical depth for electron scattering is approximately 1–10. The clumpy winds accelerated by radiation force are considered to originate from the region within <300 rg.

Venue: #220, 2F, Main Research Building (Main Venue) / via Zoom

Event Official Language: English

iTHEMS NOW & NEXT 2026

April 13 (Mon) - 14 (Tue) 2026

We will hold an annual in-house gathering, “iTHEMS NOW & NEXT,” for FY 2026. The event provides a great opportunity for all iTHEMS members, including visiting researchers and, in particular, new arrivals, to gain a comprehensive overview of iTHEMS’s current activities and future directions. The detailed program will be announced in due course, but there will be poster sessions for all members, so please be ready to present one.

Venue: 2F Large Conference Room, Administrative Headquarters, RIKEN Wako Campus (Main Venue) / via Zoom

Event Official Language: English

TJR-iTHEMS Joint Seminar: Golden Age of Neutron Stars

April 17 (Fri) 16:00 - 17:00, 2026

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

This is a TJR-iTHEMS Joint Seminar supported by ASPIRE Program ABSTRACT 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. 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 observations of heavy neutron stars of about 2.0 solar masses and higher; ongoing inferences of masses and radii by the NICER telescope; and observations of binary neutron star mergers, through gravitational waves as well as across the electromagnetic spectrum. Theoretically an understanding is emerging in QCD of how nuclear matter can turn into deconfined quark matter, which I will illustrate with modern quark-hadron crossover equations of state. BRIEF BIO Gordon Baym is a Professor of Physics at the University of Illinois. Educated at Cornell and Harvard, he spent two years at the Niels Bohr Institute. His interests range from matter under extreme conditions to ultracold atomic physics, astrophysics, and nuclear physics. A pioneer in the study of pulsars and neutron stars, he is a member of the U.S. National Academy of Sciences and received the APS Medal for Exceptional Achievement in Research, the Hans Bethe and Lars Onsager Prizes, and the Eugene Feenberg Memorial Medal.

Venue: H701, The University of Osaka, Toyonaka Campus

Event Official Language: English

iTHEMS x academist Online Event "World of Mathematical Sciences 2026"

April 18 (Sat) 10:00 - 15:30, 2026

Junnosuke Koizumi (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Osamu Fukushima (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Muzi Hong (Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Kenji Okubo (Special Postdoctoral Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Venue: via Zoom

Event Official Language: Japanese