# Coffee Meeting Log

2023-11-10

## Introduction to symmetry and band topology

Seishiro Ono (Special Postdoctoral Researcher, iTHEMS)

The last decade has seen significant advances in the understanding of topological materials. In this talk, I will briefly discuss what topological insulators are and how they can be efficiently distinguished from atomic insulators.

2023-10-27

## How to mark IMO papers

Hiroki Kodama (Visiting Scientist, iTHEMS / Research Fellow, Center for Mathematical Engineering, Musashino University)

I will report my experience in marking papers in International Mathematical Olympiad (IMO). I will explain why and how it is difficult to mark IMO papers. You can find the set of problems at https://www.imo-official.org/problems.aspx I was in charge of Problem 4., so if you are interested please try to solve it. Problem 1 and 4 are considered to be easy (as an IMO problem, of course).

2023-10-20

## Applications and Extensions of the Nielsen-Ninomiya Theorem in Condensed Matter Physics

Ching-Kai Chiu (Senior Research Scientist, iTHEMS)

Over four decades ago, Nielsen and Ninomiya presented a groundbreaking discovery showing that chiral fermions on a lattice must obey the principle of fermion doubling, ensuring an equivalent count of left-handed and right-handed fermions. This pivotal theorem has found significant applications in the realm of condensed matter physics, most notably in the study of topological states of matter. Intriguingly, a deeper exploration beyond the theorem's original scope reveals that, with the preservation of certain additional symmetries, the symmetries necessitate a minimum of more than two handed fermions on the lattice.

2023-10-13

## Introduction to attosecond physics

Hidetoshi Taya (Special Postdoctoral Researcher, iTHEMS)

The winners of the Nobel prize 2023 in physics were Pierre Agostini, Ferenc Krausz and Anne L’Huillier, who made great contributions to the foundation of attosecond physics. Although I’m don’t work exactly in this field but in something related, so I’d like to take this opportunity to briefly review the idea of attosecond physics and its relation to my studies on high-energy physics.

2023-10-06

## Enhance machine learning model in molecule science by multiscale correlation

Yaokun Lei (Postdoctoral Researcher, iTHEMS / Postdoctoral Researcher, Theoretical Molecular Science Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Correlation stands as a foundational concept in molecular dynamics simulations. It underpins various mechanisms in chemical and biological phenomena, revealing how different degrees of freedom interact to give rise to specific macroscopic events. Essentially, researchers aim to unveil the intricate structure of the n-dimensional variable space to pinpoint regions that exert significant influence on the model or observed output. Armed with this knowledge, they can construct more refined parametric models for improved prediction and control of system behavior.

2023-09-29

## A thermodynamical formalism for Kählerian spacetime

Eiji Inoue (Special Postdoctoral Researcher, iTHEMS)

When a Kähler manifold X admits a unique "nice/stable" shape (cf. Kähler-Einstein metric, cscK metric), one can imagine there is a time-dependent equation (flow) on shape which stabilizes any initial shape to the stable shape. One might speculate any X admits such a "nice/stable" shape, but it is not the case. It is then interesting to see the limit behavior (canonical boundary condition) of the flow when X does not admit "nice/stable" shape. One hopes to characterize such canonical boundary condition as a unique maximizer of some functional on the space of boundary conditions (formalized as non-archimedean metrics), independent of initial metric. I will sketch the structure of one such framework on canonical boundary condition, in which the functional is nalogous to free energy and is deeply related to Perelman entropy.

2023-09-15

## COVID-19: then, now, and into the future

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

As we are (hopefully, possibly) finally reaching the peak of Japan's 9th COVID-19 wave --- which began around mid-March 2023, when the government lifted the mask mandate --- I would like to present a perspective on the large-scale epidemic that was, still is, and what we can hope for or expect in the future. I would like to make this a very informal talk: I want you to feel free to interrupt and ask questions. I made sure not to prepare too much material to leave time for lively interactions.

2023-09-08

## A perspective on symmetries and their usefulness in our cosmos

Ryo Namba (Senior Research Scientist, iTHEMS)

2023-09-01

## New constraint on neutron star mass and radius

Hajime Sotani (Research Scientist, iTHEMS / Research Scientist, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Neutron stars are a suitable candiadte for probing the extreme states. In particular, the mass and radius constraints help us to understand the equation of state for high density matter. In this talk we show a new constraint on GRB 200415A by indentifying the observed QPOs with crustal torsional oscillations.

2023-08-25

## Exploring Quantum Spacetime

Yuki Sato (Associate Professor, National Institute of Technology, Tokuyama College)

According to Einstein’s special theory of relativity, space and time are inherently non-separable and collectively dubbed spacetime. Einstein’s another expanded theory, general relativity (GR), claims that spacetime is a dynamical entity, and the theory explains various astronomical observations very well. On the other hand, spacetime at its very beginning is supposed to be too small for GR to work properly. For such a small spacetime, quantum mechanics should play a crucial role, coming into line with GR, which may cure the situation. The spacetime that is influenced by the law of quantum mechanics is called quantum spacetime. My research is to investigate the very nature of quantum spacetime, in particular through the use of lattice discretization. In my talk, I plan to give an elementary introduction to studies of quantum spacetime.

2023-08-18

## TBD

Yasunori Nomura (Director, Berkeley Center for Theoretical Physics, University of California, Berkeley, USA)

2023-08-04

## Current status of particle dark matter

Motoko Fujiwara (Postdoctoral Researcher, Theoretical Particle Physics Group, Technical University of Munich, Germany)

In this talk, we will overview the current status of particle dark matter (DM). DM is a hypothetical matter that is believed to exist in our universe. We have discovered overwhelming evidence, such as rotational curves of the galaxies, but only through gravitational interaction. One interesting possibility is that DM can be an unknown elementary particle that interacts with the Standard Model (SM) particles. First, we will review particle DM candidates, search directions, and their latest results, through which we figure out the implications of theoretical properties of DM at the current stage. We also discuss new ideas to overcome limitations of the existing search directions and to probe unexplored DM parameter space comprehensively.

2023-07-28

## Integral Equation Approach for Solving the Boltzmann Equation

Derek Beattie Inman (Research Scientist, iTHEMS)

I will explain why integral equations can be more useful than differential equations when solving the Boltzmann equation (no knowledge of cosmology required!)

2023-07-14

## Classical simulations of quantum computers

Enrico Rinaldi (Visiting Scientist, iTHEMS / Senior Research Scientist, Quantum Machine Learning and Algorithms, Quantinuum K.K.)

Everything you wanted to know about quantum computers without using quantum computers. An introductory view of simulating quantum circuits and the current status of quantum computers.

2023-07-07

## Long-range correlation in nonequilibrium systems

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

Equilibrium states of physical systems, such as fluids and spins, typically show a short-range correlation with exponential decay, except in the symmetry-broken phase or at the critical point. In contrast, systems driven by external forces like shear flow can show a long-range correlation with power-law decay as a generic feature of nonequilibrium systems. Active matter, a crowd of self-propelled elements like bacteria or cells, is another nonequilibrium system, and its physical properties have been studied intensely. In this talk, I will overview the backgrounds and some results of our recent work that proposes a connection between externally driven systems and active matter through a long-range density correlation.

2023-06-30

## Black hole image in a nutshell

Che-Yu Chen (Special Postdoctoral Researcher, iTHEMS)

With the recent release of the black hole images from EHT, various image features have been discovered, and some of them have even been in the spotlight among the community. Starting from the definition of photon sphere, I'll introduce some of these image features, including critical curve, photon rings, and inner shadow.

2023-06-23

## Quantum Gravity and Holography

Zixia Wei (Special Postdoctoral Researcher, iTHEMS)

Quantum gravity is one of the most fundamental topics in theoretical physics. In this talk, I will sketch what quantum gravity is, why physicists are curious about it, and how the concept of "holography" plays an important role in recent studies of quantum gravity.

2023-06-16

## Scalable quantum simulation for correlated matters

Rongyang Sun (Postdoctoral Researcher, iTHEMS / Postdoctoral Researcher, Computational Materials Science Research Team, RIKEN Center for Computational Science (R-CCS))

In this talk, I would like to explain the scalable simulation of correlated quantum many-body systems on present noisy quantum devices. This talk is based on two recent works, arXiv:2210.14662 and arXiv:2303.17187.

2023-06-09

## Tensor network methods for Quantum Computing

Tomonori Shirakawa (Senior Research Scientist, iTHEMS / Senior Scientist, Computational Materials Science Research Team, RIKEN Center for Computational Science (R-CCS))

I will briefly introduce why we are interested in tensor network methods for the use of variational quantum algorithms and quantum computing.