# Coffee Meeting Log

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.

2024-07-05

## Superconductors and superfluids as macroscopic quantum condensates

Yuta Sekino (Postdoctoral Researcher, iTHEMS / Postdoctoral Researcher, Nonequilibrium Quantum Statistical Mechanics RIKEN Hakubi Research Team, RIKEN Cluster for Pioneering Research (CPR))

Superconductors and superfluids are states of matter where dissipationless transport occurs due to macroscopic manifestations of quantum mechanical effects. These states of matter appear in various physical systems, such as Bose-Einstein condensates of ultracold atomic gases, liquid helium, superconductors in solids, and probably nuclear matter inside neutron stars, attracting interest across various fields of science. In this talk, I will explain the basics of quantum condensation, the mechanism behind macroscopic quantum effects in superconductivity and superfluidity.

2024-06-28

## Y chromosome – An entity of an evolutionary dead-end?

Masafumi Nozawa (Associate Professor, Tokyo Metropolitan University)

In many organisms with genetic sex determination, sex chromosomes (X and Y or Z and W, hereafter) have emerged from a pair of autosomes. Then, the X and Y (or Z and W) stop recombination in meiosis to maintain stable sex determination, which is inevitable in many cases. Consequently, many genes on the Y are nonfunctionalized or lost due to inefficacy of natural selection. Indeed, our humans only have ~70 genes on the Y while maintaining >800 genes on the X. However, the Y is still indispensable because the Y harbors the male-determination gene. Therefore, the Y has been regarded as an evolutionary dead-end, i.e., a sandwich between two evolutionary forces: degeneration and maintenance. I will introduce the situation of the Y in several organisms.

2024-06-21

## A brief introduction to data-driven dynamical systems

Gen Kurosawa (Senior Research Scientist, iTHEMS)

Imagine that you are in a cave. The room is in a cave so that temperature and light-intensity are constant over time. Can you wake up tomorrow or day after tomorrow? In fact, most humans can wake up tomorrow and day after tomorrow almost regularly. How is it possible? To consider such a question, dynamical systems provide a mathematical framework to model interactions between quantities that evolve over time. Usually, the equations governing these systems are unknown or only partially known. Predicting and controlling such systems can be challenging. Recently, data-driven approaches have made significant strides in uncovering the equations of dynamical systems, predicting their behavior, and controlling them. In this presentation, I will review these approaches from the literature, which can be possibly applicable not only to daily rhythms but also to various other fields.

2024-06-14

## What is density functional theory?

Takeru Yokota (Special Postdoctoral Researcher, iTHEMS)

Matter in the world consists of numerous quantum particles, such as electrons and nucleons. Its properties are dictated by the Schrödinger equations for many-body systems. However, directly solving these equations poses a formidable computational challenge. Density Functional Theory (DFT), established by Hohenberg, Kohn, and Sham in the 1960s, represents one of the most successful methods for addressing many-body systems. It offers scalable approximations based on the variational principle concerning density. DFT is particularly notable for its perspective on the quantum world, wherein the ground-state density serves as an alternative to the wave function. In this talk, I will provide a brief introduction to DFT.

2024-06-07

## An intriguing property of neural networks (up to date)

Akinori Tanaka (Senior Research Scientist, iTHEMS / Senior Research Scientist, RIKEN Center for Advanced Intelligence Project (AIP))

Neural networks have played a central role in the recent development of machine learning technology, but their properties remain mysterious, and it would be interesting if these could be mathematically modeled. For example, in word embedding in a language model, it is known that "king vector" representing the word "king" appears in the learning process, and that the vector obtained by subtracting "queen vector" from "king vector" becomes a vector representing the change of words from female to male. In this talk, I would like to briefly explain that this kind of "concept arithmetic," is also possible among the weight parameters of trained neural networks, which is called "task arithmetic".

2024-05-31

## An Introduction of Room Acoustics: Theory of Reverberation Time

Shingo Gibo (Postdoctoral Researcher, iTHEMS)

It is important to predict and optimize the acoustics of a room before building the room. One of the most important indexes in the room acoustics is the reverberation time, which is defined as the time it takes for the sound energy to decay by a factor of 10^{-6}. If the reverberation time is too long, understanding speech in the room becomes difficult. Conversely, if the reverberation time is too short, we may not enjoy music in the room. In this talk, I will briefly explain the theory of the reverberation time.