Coffee Meeting Log

2025-01-10

What is Chaos?

Gabriele Di Ubaldo (Postdoctoral Researcher, iTHEMS)

Chaos is an intuitive concept yet notoriously hard to define and study by nature. I will introduce different notions of chaos that are applicable to classical and quantum systems and discuss their relations, giving examples of very different chaotic systems.

2024-12-20

Re-summing quasi-normal modes - where divergence meets our universe -

Ryo Namba (Senior Research Scientist, iTHEMS)

Series expansions provide a powerful control over problems in physics. Yet such expansions are often divergent, and it is a non-trivial task to interpret their consequences. This is, however, not something that is depressing, but rather something we should be excited about. Even though an expansion is a perturbative one, its infinite series inherits information about fully non-perturbative effects. In this short coffee meeting talk, I present one such example: quasi-normal modes of peturbations around black hole background spacetime. This is essentially to look for eigenvalues of Regge-Wheeler-Zerilli-type equations by re-summing a conventional WKB series solution, which is divergent in general, and applying proper boundary conditions.

2024-12-06

Recipe for soup of quarks and gluons

Yuuka Kanakubo (Postdoctoral Researcher, iTHEMS)

Can you imagine what the hottest soup on the earth is like? Is it viscous as honey or runny as water? How can we make such a hot soup? The hottest soup of quarks and gluons, known as quark-gluon plasma, has been studied through relativistic heavy-ion collision experiments at the Relativistic Heavy Ion Collider (RHIC) at BNL and the Large Hadron Collider (LHC) at CERN. In this talk, I will explain what is known and unknown about quark-gluon plasma, and how we study the properties with numerical simulations of relativistic heavy-ion collisions.

2024-11-29

What is “equilibrium”?

Kyosuke Adachi (Research Scientist, iTHEMS / Research Scientist, Nonequilibrium Physics of Living Matter RIKEN Hakubi Research Team, RIKEN Center for Biosystems Dynamics Research (BDR))

In interdisciplinary environments like iTHEMS, effective communication can be challenging due to differences in terminologies. In this 15-minute online talk, I will explore the concept of “equilibrium,” a term whose meaning varies across disciplines, ranging from mathematics to thermodynamics. By examining how “equilibrium” is understood in different research fields, I aim to bridge gaps in communication and foster a deeper interdisciplinary dialogue.

2024-11-22

Quark deconfinement in superdense matter: baryons or quarks? 

Yuki Fujimoto (Postdoctoral Researcher, iTHEMS)

I will explain the qualitative picture regarding the quark deconfinement in superdense matter. Naively, we expect that superdense matter, found at neutron star core densities and beyond, consists of liberated quarks rather than of confined baryons because of the asymptotic freedom. However, QCD at large Nc implies a different interesting possibility: quark confinement persists, so the system is composed of baryons, while the thermodynamics is still governed by quarks.

2024-11-15

Kinetic plasma turbulence and geometry

Motoki Nakata (Visiting Scientist, iTHEMS / Associate Professor, Faculty of Arts and Sciences, Komazawa University)

Plasmas in the laboratory and in space are often collisionless, but still exhibit collective fluid-like dynamics. Most plasmas are also turbulent, and microscopic fluctuations, vortices, and even large-scale flows develop due to nonlinear interactions. Kinetic description is an effective approach for such collisionless or weakly collisional turbulent plasmas, and the basic ideas will be introduced in this talk. In particular, I will present on the interplay between turbulent dynamics and magnetic geometry such as torus that confine the plasma. This is also a question of how turbulence and flows can be activated or deactivated by geometry.

2024-11-08

A subjective view on what is a quantum effect

Amaury Micheli (Postdoctoral Researcher, iTHEMS)

In this talk I will first sketch the difference in formalism and predictions between a classical and a quantum model of the same system.The differences will be illustrated by taking a spin-1/2 system as an example. Based on these I will give my point of view on what is a genuine quantum effect: an effect that cannot be predicted in whole class of classical theories. The landmark example of such effect is a Bell inequalities violation. Finally, I will shortly discuss the implications of the existence of such genuine quantum effect at a practical and interpretational level.

2024-11-01

Bell's inequality

Yantao Wu (Research Scientist, iTHEMS)

I will explain what Bell's inequality is without assuming you know quantum mechanics.

2024-10-25

Arrangement of cells: how are they organized in your body (or not)?

Akihisa Yamamoto (Research Scientist, iTHEMS)

Living organisms are composed of cells that adhere to each other and extracellular matrices (ECM), a complex network of proteins and biologically active molecules. Cells dynamically renew and replace each other through division and death while preserving the structure of tissues. This stability, known as homeostasis, is vital for proper tissue functions. However, diseases such as cancer can disrupt the typical arrangement of cells in tissues. Although there is some empirical understanding of the relationship between tissue structure and disease state, where skilled doctors can diagnose by examining tissue images, it is still largely unknown why tissue structure gets altered according to disease progression and what aspects of the physiological and mechanical changes of cells and tissues are responsible for it. In my talk, I will present examples of how disease affects tissue structure and demonstrate how we can characterize these changes.

2024-10-18

Mathematical aspects of the entropic uncertainty relation

Takashi Satomi (Special Postdoctoral Researcher, iTHEMS)

The uncertainty relation is originally a fundamental concept in quantum mechanics. This relation limits the precision with which two physical quantities (such as position and momentum) can be simultaneously known. In mathematical aspects, the uncertainty relation implies that either the original function or its Fourier transform has a spread graph for any function. In this talk, we see a variant of the uncertainty relations by using the entropy, which is called the entropic uncertainty relation. In addition, we observe that this is stronger than the variance version (and hence the standard deviation version) of the uncertainty relation.

2024-10-11

On mixed-state topological phases

Osamu Fukushima (Special Postdoctoral Researcher, iTHEMS)

Topological phases are quantum phases of matter characterized by the robustness against local perturbations. While the understanding of topological phases in pure states has reached a mature stage over the past few decades, their mixed-state counterparts remain less explored. In this talk, I will review the concept of symmetries for mixed states, which is essential for characterizing mixed-state topological phases, and introduce an intrinsically mixed-state topological phase that does not belong to the same equivalence class as any pure states.

2024-09-27

Quadratic residues and domino tilings

Junnosuke Koizumi (Special Postdoctoral Researcher, iTHEMS)

Quadratic residues are a classical concept in number theory, with a history tracing back to the times of Fermat and Euler. On the other hand, the number of ways to tile a rectangular grid with dominoes was given an explicit formula by Kasteleyn and Temperley-Fisher in the 1960s, in the context of research on the dimer model. In this talk, I will introduce recent research by myself, Y. Kamio, and T. Nakazawa, which connects these two concepts.

2024-09-20

The correspondence between classical and quantum mechanics via pseudodifferential operators

Keita Mikami (Research Scientist, iTHEMS)

In this talk, I will briefly explain how classical mechanical time evolution and quantum mechanical time evolution are related using pseudodifferential operators.

2024-09-13

Gauge field topology on the lattice

Okuto Morikawa (Special Postdoctoral Researcher, iTHEMS)

Lattice gauge theory is the most-established framework of non-perturbative quantum field theory (QFT). Nevertheless quite many structures in continuum QFT become sacrifices to gauge invariance. We would like to discuss topology of lattice gauge fields; it is an essential construction of gauge theory and leads us to an important observation of recent generalization of symmetry.

2024-09-06

Operator algebras meet number theory

Kan Kitamura (Special Postdoctoral Researcher, iTHEMS)

Operator algebras are algebras consisting of linear operators, which may be interpreted physically as observables. Their study is interesting on its own, forming a branch of mathematics. We will glimpse an occasion where the theory of operator algebras meets number theory, a different branch of mathematics investigating integers. Such an occasion sometimes emerges when we consider quantum symmetry, a kind of symmetry beyond usual groups. We will try to sketch a rough overview of how these concepts interact.

2024-08-30

Crystal space group

Congcong Le (Postdoctoral Researcher, iTHEMS)

In condensed matter physics, the space group is a fundamental theoretical tool for studying the physical properties of crystalline materials, providing a systematic way to understand and predict their behavior. We start with two-dimensional crystal structures, introducing the concepts of rotational and mirror symmetries, which then lead to the definition of point groups and space groups. The discussion is then extended to three-dimensional lattices. Finally, we discuss the application of space groups in electronic structure.

2024-08-23

Towards the unification of the speciation

José Said Gutiérrez Ortega (iTHEMS)

Speciation, the process by which new species originate, occurs due to geographic (physical distance), ecological (different background environments), and historical (divergence time) factors that promote reproductive isolation among lineages. However, we don’t know how these factors interplay; therefore, our empirical and theoretical knowledge about speciation is limited, fragmented, and lacks unification. To fill this knowledge gap, I propose a model and an experiment that treats speciation as a continuum of the interplay between geographic and ecological factors. Empirical evidence has shown that the extremes of this continuum produce high speciation rate (faster speciation), while I expect that intermediate values in the interplay continuum would produce reduced speciation rates. Because my theoretical background is not strong, I would like to receive your feedback about the feasibility of this model and about the accuracy of these expectations. Your comments are greatly appreciated.

2024-08-02

Toward the Quantum Theory of Gravity

Yasunori Nomura (Senior Research Scientist, iTHEMS / Director, Berkeley Center for Theoretical Physics, University of California, Berkeley, USA)

TBD

2024-07-26

Infrared Triangle

Puttarak Jai-akson (Postdoctoral Researcher, iTHEMS)

In the low-energy (infrared) regime, there exists a three-way relationship among three key concepts in physics within asymptotically flat spacetime: asymptotic symmetries, soft theorems, and memory effects. In this talk, I will explore these relationships and their significance.

2024-07-12

Less could be more - diagonalize matrices

Tsukasa Tada (Deputy Program Director, iTHEMS)

When we handle matrices, which are ubiquitous, especially in quantum physics and information theory, we try to diagonalize them. While diagonalized matrices exhibit familiar features with ordinary numbers, such as commutativity, they sometimes elude our intuition. In this talk, I introduce a new approach that has recently attracted much attention: one that handles matrices by not totally diagonalizing but leaving two upper and lower adjacent elements to the diagonal elements non-zero, which yields tridiagonal matrices.