Seminar

Understanding Biological Clocks Using Methods from Applied Mathematics and Theoretical Physics

April 16 (Thu) 12:30 - 13:30, 2026

Gen Kurosawa (Senior Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Imagine that you are in a room with no information about time. The room is located in a cave, where temperature and light intensity remain constant. In such an environment, would you be able to wake up tomorrow or the day after? In fact, most humans can wake up at roughly similar times on successive days. This is because we possess internal daily rhythms, known as circadian rhythms. Biological experiments have shown that such rhythms are not unique to humans, but are shared by many species on Earth. In this talk, I will introduce some open problems related to these daily rhythms, and discuss approaches based on dynamical systems theory and the renormalization group method, from the perspectives of applied mathematics and theoretical physics.

Venue: via Zoom / Seminar Room #359

Event Official Language: English

From Classical Definiteness to Geometric Predictability: Complementarity, Coherence, and Thermodynamic Triality

April 10 (Fri) 15:30 - 17:00, 2026

Ezra Acalapati Madani (Ph.D. Student, Laboratoire de Physique de l'École Normale Supérieure, France)

Wave–particle complementarity is one of the central principles of quantum mechanics, traditionally quantified through the Englert–Greenberger–Yasin relation between which-way information and interference visibility. In higher-dimensional and resource-theoretic settings, however, visibility is no longer unique, and it becomes natural to reformulate complementarity in terms of basis-dependent predictability, coherence, and mixedness. In this talk, I present two related works along this line. First, I discuss an exact complementarity relation between classical definiteness and quantumness, where definiteness is defined operationally through the resilience of a quantum state under nonselective dichotomic yes/no measurements, while the complementary quantum contribution is quantified using a Kirkwood–Dirac-based notion of coherence/interference motivated by recent KD-based coherence measures. Second, I introduce a geometric predictability defined by the Bures distance between the dephased state and the maximally mixed state. This predictability depends only on the observed measurement statistics and admits a closed form in terms of the Bhattacharyya overlap. For pure states, it satisfies an exact complementarity relation with nonclassical Kirkwood–Dirac coherence; for mixed states, this motivates a convex-roof extension whose operational meaning is the classically irreducible part of measurement randomness, with implications for guessing probability and min-entropy. Finally, motivated by the decomposition of entropy production into population and coherence contributions in quantum thermodynamics, and by standard wave–particle–mixedness triality relations, I show how the usual predictability–coherence duality can be promoted into a triality relation involving predictability, coherence, and mixedness. Altogether, the talk connects wave–particle duality, coherence resource theories, operational guessing tasks, and thermodynamic balance relations within a unified framework.

Venue: #359, Seminar Room #359

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

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

Quantum Computation SG seminar 2026

April 7 (Tue) 15:00 - 17:00, 2026

Self-introductions (name + research interests) and discussion about study group activities in FY2026.

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

Event Official Language: English

Physics-based eruption forecasting at Kīlauea volcano using an Ensemble Kalman Filter

April 7 (Tue) 13:00 - 14:30, 2026

Kyle R. Anderson (Research Geophysicist, California Volcano Observatory, U.S. Geological Survey (USGS), USA)

Today, most forecasts of volcanic eruptions are based on expert opinion, making them fundamentally subjective. Such forecasts have often proven successful but have clear limitations. Novel quantitative forecasting techniques have shown promise in experimental settings (hindcasting) but face numerous operational challenges and most have rarely if ever been applied to real-world eruptions (forecasting). In this talk I will discuss efforts to forecast a remarkable ongoing series of more than 40 high lava fountain eruptions at Kīlauea volcano, Hawaii, using a simple physics-based model in an Ensemble Kalman Filter (EnKF) data assimilation algorithm. Using this method, which is believed to be the first implementation of a physics-based EnKF eruption forecast, the times of Kīlauea’s lava fountain eruptions can be forecast days to weeks in advance. The method assimilates geodetic data to constrain the evolving state of the system, provides insight into the eruption mechanism and rate of magma supply to the volcano, and produces fully probabilistic forecasts. These forecasts are combined with other information, including forecasts based on machine learning algorithms, to derive forecast windows, which are disseminated to the public and to partner agencies for hazards mitigation activities. In this way, novel eruption forecasting tools are continually developed which serve an important public need while also improving understanding of the volcanic system.

Venue: Hybrid Format (RIKEN R-CCS room C107 and 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

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. Themethod 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 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

Satellite Data Assimilation for Numerical Weather Prediction (NWP)

March 31 (Tue) 14:30 - 16:00, 2026

Martin Weissman (Professor, Department of Meteorology and Geophysics, University of Vienna, Austria)

Satellite data assimilation for NWP has made tremendous progress over the past decades. Most of the assimilated observations in global NWP systems are nowadays satellite radiances from passive sensor in the infrared and microwave range. Additionally, GPS radio occultation provides information on upper-level humidity that serves as important uncalibrated anchoring information for humidity. Nevertheless, there are still significant limitations, especially in terms of lacking wind information that controls atmospheric dynamics in global NWP as well as in terms of using cloud-affected radiances in regional, convection-permitting NWP. My presentation will cover recent progress of my group in these fields.

Venue: Hybrid Format (RIKEN R-CCS room C107 and Zoom)

Event Official Language: English

Ecological decline and biocultural loss in Cycas revoluta landscapes of the Amami Islands

March 31 (Tue) 13:00 - 14:00, 2026

Joshua Englehardt (Professor, Center of Archeologist Studies, El Colegio de Michoacán, Mexico)

Cycads (Cycadales) are one of the world’s most ancient plant lineages, and Cycas revoluta Thunb. (‘sotetsu,’ in Japanese) has long occupied a central place in the cultural ecologies of the Ryukyu archipelago, particularly in the Amami Islands of southern Japan. Although never domesticated, C. revoluta has held enduring alimentary, ethnoecological, and symbolic saliency within local agroecological systems, ritual landscapes, and island identities for centuries. Building on recent interdisciplinary scholarship on Japanese and Ryukyuan cycad cultures, this presentation synthesizes ethnobotanical, historical, ecological, and genetic research to detail the accelerating collapse of Amami cycad biocultural heritage. The core of this talk focuses on results from ongoing fieldwork documenting the rapid spread of cycad aulacaspis scale (Aulacaspis yasumatsui Takagi), an invasive insect that now poses an existential threat to both biological C. revoluta populations and sotetsu culture across the Amami archipelago. Drawing on systematic botanical surveys, geospatial mapping, genetic sampling, and ethnographic interviews, the presentation details how ecological decline and cultural erosion are unfolding in tandem. Population-level mortality, reproductive failure, and genetic loss are paralleled by the disappearance of knowledge, practices, and senses of place historically anchored in the islands’ cycad landscapes. By situating these findings within broader discussions of cycad use in Japan and worldwide, as well as comparative biocultural heritage studies, the presentation highlights how invasive species can rapidly destabilize long-standing human-plant relationships. The Amami case underscores the urgency of integrating biological conservation with cultural documentation at moments of irreversible ecological change, offering broader insights into island resilience, heritage loss, and the fragility of biocultural systems under accelerating environmental pressures.

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

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

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

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

Data-Driven Stratification and Prediction of Complex Diseases

March 24 (Tue) 14:00 - 15:15, 2026

Eiryo Kawakami (Team Director, Medical Science Data-driven Mathematics Team, Division of Applied Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Many common diseases such as cancer, chronic heart failure, and diabetes exhibit substantial biological and clinical heterogeneity, which complicates diagnosis, risk assessment, and treatment decisions. In this talk, I introduce a data-driven framework for disease stratification and prediction using machine learning applied to multidimensional medical data. First, unsupervised machine learning methods are used to identify previously unrecognized disease subtypes based on clinical and biomarker data. These stratification approaches reveal hidden patient groups with distinct clinical characteristics and prognoses. To enable practical application in clinical datasets, we further develop supervised learning models that reproduce and generalize unsupervised clusters, allowing robust subtype estimation even in datasets with missing variables. Next, I present approaches for early disease detection using large-scale medical history data, focusing on combinations of comorbidities as early indicators of severe diseases. Finally, I discuss how large-scale deep learning models can be leveraged to predict disease prognosis from medical images and other high-dimensional data. These studies demonstrate how machine learning can redefine disease categories and enable earlier detection and more precise prediction in heterogeneous diseases.

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

Event Official Language: English

Mouse Limb Bud Skeletal Patterning Description and Modelling

March 19 (Thu) 13:00 - 14:00, 2026

Laura Aviñó Esteban (Ph.D. Candidate, European Molecular Biology Laboratory, Barcelona, Spain)

Understanding how complex organs reliably form during development remains a key question in biology. In this talk, I discuss how gene regulatory networks may generate skeletal patterns in the vertebrate limb, using Sox9 expression as a proxy, as it marks the earliest stages of cartilage formation. To address this, I developed new computational tools for reconstructing spatiotemporal gene expression and built models ranging from machine learning approaches to mechanistic frameworks. These analyses reveal that limb patterning cannot be explained by a single universal mechanism. Instead, different regions of the limb appear to use distinct regulatory strategies, uncovering an unexpected qualitative modularity in skeletal development. Together, these findings lead to a new hypothesis in which other systems, such as the vasculature may actively shape skeletal spacing in specific limb regions.

Venue: via Zoom / Seminar Room #359

Event Official Language: English

Critical Phenomena on the Bethe Lattice

March 18 (Wed) 16:00 - 18:00, 2026

Saswato Sen (Ph.D. Student, Okinawa Institute of Science and Technology Graduate University (OIST))

We investigate the critical behavior of a family of Z2-symmetric scalar field theories on the Bethe lattice (the tree limit of regular hyperbolic tessellations) using both the non-perturbative Functional Renormalization Group and perturbation theory. Due to the hyperbolic nature of Bethe lattices, the Laplacian lacks a zero mode and exhibits a spectral gap. We demonstrate that closing the spectral gap via a modified Laplacian leads to novel critical behavior governed by interacting fixed points. This stands in contrast to the nearest-neighbor Ising model, which exhibits a phase transition with mean-field critical exponents. We further comment on the possible reasons for such a deviation.

Venue: via Zoom / #359, Main Research Building

Event Official Language: English

Testing the quantum nature of gravity "ab absurdo"

March 18 (Wed) 14:00 - 16:00, 2026

Emanuele Panella (Postdoctoral Researcher, Tor Vergata University of Rome, Italy)

The quest for a quantum description of gravity has been long, diverse, and productive. Yet, despite decades of theoretical progress, there is still no direct experimental evidence for the quantum nature of spacetime. In this talk, I explore an alternative, indirect route to probing quantum gravity by assuming the fundamental classicality of the gravitational field and examining the resulting observational conflicts. In particular, I will discuss a key consistency condition—known as the decoherence–diffusion trade-off—that any theory of fundamentally classical gravity coupled to quantum matter must satisfy. By analysing a toy model of a linearised classical–quantum (CQ) gravity–matter system, I will explicitly show how this trade-off implies unavoidable, measurable effects, such as a fundamental stochastic gravitational-wave background, which cannot be eliminated by fine-tuning the model parameters.

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

Event Official Language: English

Quantum modular form and quantum invariants

March 13 (Fri) 14:00 - 16:00, 2026

Yuya Murakami (Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Quantum invariants are invariants of knots and 3-manifolds which relate deeply to mathematical physics and representation theory. In recent years, it has become increasingly clear that it is also deeply related to number theory, that is, quantum modularity for quantum invariants. This topic is interesting from a topological viewpoint since this is a refinement of establishing asymptotic expansions of quantum invariants, which is an important problem in quantum topology, and is interesting from a number-theores[tic viewpoint since this gives examples of quantum modular forms, which are mysterious objects in number theory. I obtained two linked results on topology and number theory: Establishing explicit asymptotic expansions of quantum invariants for negative definite plumbed 3-manifolds and establishing quantum modularity of false theta functions in full generality. In this talk, I will outline previous progress on quantum modularity for quantum invariants and my results.

Venue: via Zoom / Seminar Room #359, Seminar Room #359

Event Official Language: English