# Coffee Meeting Log

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 / Postdoctral 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.

2023-05-26

## Introduction to quotient spaces

Kazuki Kannaka (Special Postdoctoral Researcher, iTHEMS)

The operation of taking a "quotient" often appears in mathematics. It is an operation to construct more complex objects from mathematical objects. In this coffee talk, I will give an introduction to "quotient" in geometry.

2023-05-19

## Theory of diffusive shock acceleration

Hirotaka Ito (Research Scientist, iTHEMS / Research Scientist, Spin isospin Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Particle acceleration is an ubiquitous physical process in the Universe. Diffusive shock acceleration (DSA) is widely accepted as the predominant mechanism responsible for particle acceleration in various astrophysical phenomena. This theory postulates that the energy distribution of the accelerated particles conforms to a power-law form characterized by E^{-2}. In this 15-minute talk, I will outline the derivation of this power-law index of -2 within the theory of DSA.

2023-04-28

## Look up the sky at multiwavelength

Naomi Tsuji (Visiting Scientist, iTHEMS / Assistant Professor, Faculty of Science, Kanagawa University)

Light (i.e., photons or electromagnetic waves) provides us with important information of sources which emit the light. Measurements of light from the sky/Universe at different wavelengths can open up a different fields of physics. While my major is observations of high-energy (X-ray or gamma-ray) photons from astrophysical sources at the distance, since I moved to Kanagawa University I also work for optical measurements of nearby stars by using a telescope at the university. I will briefly introduce my current works at the coffee meeting.

2023-04-21

## Discrete approximation to the Laplacian

Keita Mikami (Research Scientist, iTHEMS)

In this coffee talk, I will introduce recent results on the approximation of Laplacian by discrete operators. Very modestly, the differential is a limit of difference. This modest picture is not always true if we consider differential operators. However, if one considers the norm resolvent convergence, one can show that we can approximate Laplacian by second-order difference operators. Refferences: Nakamura, S., Tadano, Y.: On a continuum limit of discrete Schrödinger operators on square lattice. J. Spectr. Theory 11 (2021), no. 1, 355-367.

2023-04-14

## When a Cretan says all Cretans are liars

Tsukasa Tada (Coordinator, iTHEMS / Vice Chief Scientist, iTHEMS)

The famous phrase "All Cretans are liars" said by a Cretan philosopher poses a paradox. This millenia-old paradox lies at the core of Goedel's incompleteness theorem. Recently this paradox has been discussed in the physics literature. I will introduce this recent interest and share my own thoughts.

2023-03-24

## The introduce of twisted bilayer graphene

Congcong Le (Postdoctoral Researcher, iTHEMS)

Graphene, a two-dimensional material possessing Dirac points, is recognized as a topological semimetal. Twisted bilayer graphene, a configuration produced by rotating two layers of graphene relative to each other, can host the energy flat bands contributed by Dirac points. These flat bands serve as a fruitful playground for investigating strong correlations and topological phases, and hence the study of twisted bilayer graphene has brought excitement and impacts to the condensed matter physics community. In this coffee meeting, I will present the introduction of twisted bilayer gaphene.