コーヒーミーティング過去ログ

2026-04-10

The transition to synchronization: from phase reduction to the Kuramoto model

リッカルド・ムオロ (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 基礎科学特別研究員)

Synchronization is one of the most studied phenomena in the field of complex systems and a paradigmatic example of self-organizing behavior. The synchronization of coupled oscillators was first observed by Christiaan Huygens, who in the 17th century noticed that pendula hanging on the same wall tend to synchronize in anti-phase. Later research highlighted the occurrence and relevance of synchronization in many natural and artificial systems, ranging from fireflies’ blinking and frogs’ croaking to rhythmic contractions in cardiac cells and neuronal activity, and from bridge oscillations to power angles in electrical power grids. The key factor enabling this collective behavior is the interaction among oscillators, which can be, for example, mechanical (bridges, pendulum clocks), electrical (heart, power grids), visual (fireflies), or acoustic (frogs). Until about sixty years ago, it was unclear how synchronization emerges. A major step forward came from Winfree, who proposed describing oscillators using only their phases and conjectured that a transition to synchronization would occur above a critical coupling strength. This idea inspired Kuramoto, who introduced his celebrated model and provided a solution to this longstanding problem. In this short talk, I will introduce the phase reduction approach and retrace the path that led Winfree and Kuramoto to these results. I will then discuss the Kuramoto model and illustrate the transition to synchronization.

2026-03-27

Condensation Tensor Network

大森 寛太郎 (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 上級研究員)

A matrix is an array of numbers with two indices, composed along a line. A tensor has more indices, and tensors can be composed along a network that fills a region of space — much like how data, signals, or physical states are organized spatially in many areas of science. What conditions on the individual tensors guarantee that the network as a whole has nice properties? I will introduce the notion of a condensation tensor network, where simple algebraic conditions on each tensor — checkable locally — ensure that the entire network behaves coherently at any scale. The resulting structure defines a projection onto a subspace that is independent of the system size. These networks provide a unified language for topological phases of matter and quantum error correction.

2026-03-13

Dipole symmetry from anomaly

戎 弘実 (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 研究員)

Anomalies often signal that a quantum system cannot realize a trivial symmetric phase. In lattice systems, certain combinations of internal and spatial symmetries lead to such anomaly constraints. We discuss that dipole symmetry, a spatially modulated symmetry, can naturally emerge from these anomaly structures. Starting from anomalous lattice models, we demonstrate that gauging part of the internal symmetry leads to the appearance of dipole symmetry in the resulting theory. This perspective provides a simple way to understand the origin of modulated symmetries and highlights how anomaly considerations can give rise to unconventional symmetry structures in quantum lattice systems.

2026-03-06

How many sequences of games are possible in Koshien?

孔 星植 (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 研究員)

I address the question “How many sequences of games are possible in Koshien?” using combinatorial techniques from mathematical phylogenetics. I model the tournament as a labeled evolutionary history and apply tree enumeration methods to count the number of distinct outcome sequences. By interpreting each game as an internal node in a rooted labeled history, the problem reduces to counting labeled histories consistent with the tournament structure. I start the talk with basic introduction of evolutionary trees and try to establish a connection between single-elimination tournaments and the combinatorics of evolutionary trees.

2026-02-27

What Are Modular Forms?

村上 友哉 (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 研究員)

In this talk, I introduce the concept of modular forms and explain some of their surprising applications to number theory, topology, and mathematical physics. Modular forms are holomorphic functions characterized by rich symmetry properties. At first glance, their definition appears purely analytic.Nevertheless, they contain deep arithmetic information and play a central role in modern number theory. In recent developments, modular forms have also emerged in the context of topological quantum field theory. This talk aims to provide an accessible overview of these ideas for a general audience.

2026-02-20

How a player can unilaterally control payoffs in repeated games

村瀬 洋介 (理化学研究所 数理創造研究センター (iTHEMS) 数理展開部門 数理社会科学チーム チームディレクター / 理化学研究所 計算科学研究センター (R-CCS) 離散事象シミュレーション研究チーム 上級研究員)

In repeated interactions, players can choose their actions based on past outcomes, leading to a wide variety of possible strategies. In this talk, I introduce "zero-determinant" strategies, discovered by the famous physicist Freeman Dyson, which show that in repeated games a player can unilaterally enforce linear relations between long-run average payoffs, independent of the opponent's strategy. I will explain the basic idea using a Markov-chain formulation of the repeated Prisoner’s Dilemma.

2026-02-13

Failure of Scale Separation in Physics

ウェイシャン・シャオ (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 基礎科学特別研究員)

The success of modern physics relies heavily on the principle of scale separation, which allows complex systems to be described by simplified effective theories that are largely insensitive to microscopic details. In this talk, I will review this Wilsonian framework and illustrate its usefulness across a wide range of physical contexts. I will then discuss situations in which this separation of scales breaks down, focusing on examples motivated by quantum gravity as well as by certain physical settings in our universe. These examples show how high-energy physics can have unexpectedly strong effects at large scales.

2026-02-06

On how to detect selection in a set of genomes

アイナ・コロメ ビラパナ (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 数理遺伝学理研ECL研究ユニット 特別研究員)

How do organisms adapt and evolve? Population genetics provides us with the theoretical framework to answer this question. A chance mutation can give rise to an advantageous mutation in one individual, but how does this mutation then spread in a population? How quickly does a mutation increase in frequency, what role does chance play in this process, and how does it impact the diversity of our genomes? This talk will introduce how natural selection appears in our genomes and how we can detect it in genetic data. I will introduce the main concepts behind some of the classical selection statistics developed early in the field. I will then show how Ancestral Recombination Graphs have revolutionized our ability to infer selection in recent years. One of the ongoing debates around selection is the extent to which genomes have been (or are currently being) shaped by this force, and what this means for how organisms adapt.

2026-01-30

Chaos bound and thermalization

福島 理 (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 基礎科学特別研究員)

Chaos refers to complex dynamical behavior, where the long-time behavior becomes effectively unpredictable even though the underlying equations of motion are deterministic. On the other hand, thermalization concerns the relaxation of observables toward equilibrium, which is a pivotal dynamical phenomenon in many-body systems. Chaos and thermalization share some common aspects, and their relation has been extensively discussed. In this talk, I will introduce how the eigenstate thermalization hypothesis, which is a central criterion for thermalization, is related to a universal bound on chaos in quantum systems.

2026-01-23

Evolution of Evolution?: Modifier Theory as a First Step

大窪 健児 (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 基礎科学特別研究員)

Evolution successfully explains how populations change across generations, but it usually treats key parameters such as mutation and recombination rates as fixed. In this talk, I introduce modifier theory as a first step toward a theory of the evolution of evolution, where these parameters themselves can evolve. Using simple finite-population toy models, I show that mutation-rate modifiers can be favored not by higher fitness, but by producing beneficial mutations earlier. This leads to history-dependent evolutionary outcomes driven by timing rather than averages. I conclude by discussing implications for mutator evolution in bacteria, cancer dynamics, and the possibility of inferring past environments from present-day evolutionary parameters.

2026-01-16

Recent progress in understanding quantum black holes

宮地 真路 (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 上級研究員)

Black holes are ubiquitous in our universe, yet their microscopic structure is still far from being understood. In particular, they pose serious theoretical puzzles, such as the black hole information paradox and the firewall problem. Recent progress based on holography has shed new light on these issues, as well as on the low-temperature quantum dynamics of black holes, connecting black holes with strongly chaotic quantum systems. In this coffee meeting, I will explain the key ingredients needed to understand these recent and fascinating theoretical developments.

2026-01-09

Many-faceted KP hierarchy

テツ・オウ (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 研究員)

The Kadomtsev-Petviashivili (KP) hierarchy is an integrable hierarchy of partial differential equations generalizing the well-known KdV hierarchy describing the dynamics of shallow water. The KP theory is extremely rich, and it relates different fields of mathematics. In this talk, we will focus on Sato’s theory for KP hierarchy and interpret it using analytic/geometric/algebraic languages.

2025-12-19

Brunn-Minkowski inequality and the Borell-Brascamp-Lieb inequality

里見 貴志 (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 基礎科学特別研究員)

The Brunn-Minkowski inequality (or Brunn-Minkowski theorem) is one of the fundamental geometric inequalities in Euclidean space. This inequality states that the volume of the sum of two sets (called the Minkowski sum) can be bounded from below in terms of the volumes of the two original sets. There are several proofs of this inequality, and one approach is to establish a functional extension known as the Borell–Brascamp–Lieb inequality. In this talk, we will review these inequalities and briefly indicate how the Borell–Brascamp–Lieb inequality may be proved by mathematical induction.

2025-12-12

Aspects of Quantum Resonance Revisited

森川 億人 (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 基礎科学特別研究員)

Quantum resonances appear in many areas of physics, yet their definitions—through scattering peaks, S-matrix poles, or non-Hermitian eigenvalues—often seem unrelated. In this talk, I revisit these aspects from a unified viewpoint: resonances as pseudo-bound states defined by analytic continuation into the complex plane. This perspective clarifies how methods such as complex scaling, Gamow states, and semiclassical tunneling fit together. I will briefly show how exact WKB naturally captures these structures, without assuming any background in resurgence theory.

2025-12-05

Graphs, Linear Algebra, and the "Champagne Glass Problem"

小泉 淳之介 (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 基礎科学特別研究員)

When people clink glasses in a toast, it is surprisingly hard for four or more glasses to all touch each other at the same time. What is the largest number of people whose glasses can be arranged so that every pair of glasses is in contact simultaneously? If we model the glasses as infinite cylinders, this turns into an open problem in mathematics. In this talk I will present an approach to this problem that uses tools from graph theory and linear algebra.

2025-11-28

Distances and Durations: Cosmology 101

アモリ・ミケリ (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 特別研究員)

Cosmology aims to describe the evolution of the Universe and its contents. The light we receive from distant objects serves as our primary observable for understanding this evolution. I will outline how this light enables us to infer the distance of its source and the time of its emission. These considerations swiftly lead to a discussion of the expansion of space, which is the main driver of the Universe's evolution and lies at the heart of many active questions in cosmology, such as the Hubble tension and the nature of dark energy.

2025-11-21

Lower bound on ground state energy in quantum chemistry

小野 清志郎 (理化学研究所 数理創造研究センター (iTHEMS) 国内外連携・人材育成部門 客員研究員 / 東京大学 物性研究所 物性理論研究部門 助教)

Thanks to the variational principle, we can obtain a meaningful upper bound on the ground-state energy if we have a good variational state. Here, I will review how to obtain a lower bound on the ground-state energy, in contrast to the variational approach. In fact, this problem can be formulated as a well-known optimization problem.

2025-11-14

Symmetries of gravity

プトゥラク・ジャイアクソナ (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 特別研究員)

Symmetry lies at the foundation of modern physics, and gravity offers a unique setting where symmetry and geometry deeply intertwine. This talk will trace how gravitational symmetries evolve: from exact isometries of spacetime to asymptotic symmetries at infinity, and ultimately to the broad class of corner symmetries defined on finite surfaces.

2025-11-07

Hilbert space fragmentation: breakdown of quantum thermalization

関野 裕太 (理化学研究所 数理創造研究センター (iTHEMS) 数理展開部門 量子数理科学チーム 特別研究員 / 理化学研究所 開拓研究所 (PRI) 濱崎非平衡量子統計力学理研白眉研究チーム 特別研究員)

Thermalization is a ubiquitous phenomenon in nature, ranging from the early universe in cosmology to electron systems in condensed matter physics. However, its understanding based on microscopic dynamics by quantum mechanics is still not so clear. In this talk, I will introduce the microscopic mechanism called Hilbert space fragmentation, which leads to dynamics violating thermalization.

2025-10-31

Graphs and quantum groups

北村 侃 (理化学研究所 数理創造研究センター (iTHEMS) 数理基礎部門 基礎科学特別研究員)

I will introduce a seemingly classical problem in graph theory that I encountered through a motivation from (relatively modern) quantum group theory. The problem is to find examples of certain strongly regular graphs: collections of nodes and edges having exceptional combinatorial properties, which will be illustrated in the talk.