# Coffee Meeting Log

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.

2024-05-24

## Unique Characterizations of Thermodynamic Entropy

Yuki Yokokura (Senior Research Scientist, iTHEMS)

Entropy has special properties related to heat, microscopic degrees of freedoms, and macroscopic irreversibility. I will explain that these are connected each other through dynamics, and add a new characterization: symmetry of entropy.

2024-05-17

## Weather forecast and deep learning

Shigenori Otsuka (Research Scientist, iTHEMS / Research Scientist, Data Assimilation Research Team, RIKEN Center for Computational Science (R-CCS))

In this talk, I will introduce application of deep learning to weather forecasting. Recent years, tech companies, such as NVIDIA, Huawei, and Google, reported their deep learning-based global weather prediction models. These models were trained on so-called atmospheric reanalyses to emulate computationally demanding numerical weather prediction models. Although we still need physically-based models for various purposes, deep learning may change the future of weather predictions.

2024-05-10

## Forming black holes from stars

Lucy McNeill (Postdoctoral Researcher, iTHEMS)

Black holes which are the remnants of stars are being detected at a rate of a few per month using various optical telescopes and gravitational wave interferometers. They provide fruitful opportunity to test and challenge stellar evolution theory, which depends sensitively on our understanding of general relativity, quantum mechanics, particle physics and nuclear physics. In this coffee talk, I will present the physical concepts and mathematical scalings behind the formation of a black hole, after a star’s iron core collapses. I will quantify the key length, time and energy scales involved, and the (possibly surprising) importance of neutrino transport.

2024-04-26

## Exotica in Mathematics

Taketo Sano (Special Postdoctoral Researcher, iTHEMS)

Since the discovery of an exotic 7-dimensional sphere by J. Milnor in 1956, the study of exotic phenomena has become one of the central topics in topology. Here, an exotic sphere is a smooth manifold that is homeomorphic, but not diffeomorphic, to the standard sphere. In this talk, I will briefly explain the history of the discoveries of exotic phenomena and discuss some of the recent achievements related to knot theory.

2024-04-19

## A gentle introduction to fluid turbulence

Camilia Demidem (Research Scientist, iTHEMS)

Turbulence is everywhere around us, manifesting itself in seemingly trivial aspects of daily life, such as the act of pouring milk into coffee, while also shaping critical processes in fusion reactors, atmospheric dynamics and astrophysical phenomena. In this talk, I will try to review some fundamental aspects of turbulence and explain why it is so challenging to model it.

2024-04-12

## Singularity Theorems

Shigehiro Nagataki (Deputy Program Director, iTHEMS / Chief Scientist, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

I am happy to introduce the Singularity Theorem, which was proved by Roger Penrose in 1965. He won the Nobel Prize in Physics in 2020 by the proof. I hope you will feel the outline of the proof and understand that the Singularity corresponds to a point outside of spacetime. If there is 1 minute left in my presentation, I would like to mention that Roger Penrose and Stephen Hawking proved the existence of (a) singularity(ies) in natural conditions at the beginning of our universe. Einstein's equation for general relativity is a kind of God's equation, but singularity theorems strongly suggest the limitation of general relativity. I want to thank Prof. Fujikawa, who requested that I give a presentation on the Singularity Theorems. His request motivated me to prepare for my presentation (originally, I was planning to give a short talk on stellar physics using a part of my notebook that I used in my lecture course at OIST).

2024-03-29

## Quantum channel characterization

Shunji Matsuura (Visiting Scientist, iTHEMS / Senior Researcher, Hardware Inovation Lab, 1QBit, Canada)

The greatest challenge in building a quantum computer is noise. Suppressing noise in quantum systems is extremely difficult, which has led to a long-standing skepticism about the feasibility of quantum computers. So, what exactly is noise in the context of quantum computers? How is it characterized, and how is it measured? In this talk, we will discuss the nature of noise and, as specific examples of methods for characterizing it, we will talk about randomized benchmarking and tomography.

2024-03-22

## ORCID to auto-report your research contributions & manage your online visibility

Catherine Beauchemin (Deputy Program Director, iTHEMS / Professor, Department of Physics, Toronto Metropolitan University, Canada)

We all have to report our papers (and grants) activities at least once per year to the institutions we work for. Different employers have different systems (e.g. RIKEN has RARS) and it is tiresome to fill these forms again and again. We also want to share or make this information visible to the wider scientific community, as part of looking for a new jobs or for new collaborators. Again a lot of different tools and databases exist (Scopus, Web of Science, Dimensions, Google Scholar, ResearchGate, Pubmed, etc.). In this talk I want to tell you about ORCID: what it is and what it can do for you. Especially, how it can help you solve the problems of reporting and widely disseminating your research accomplishments to the community across the different platforms, while managing it in just one place: your ORCID record. I'll demonstrate some of nice applications.