# コーヒーミーティング過去ログ

2023-02-03

## Quantum decoherence and the Caldeira-Leggett model

横田 猛 (数理創造プログラム 基礎科学特別研究員 / 東京大学 物性研究所 特別研究員)

Quantum decoherence is one of the fundamental phenomena of quantum systems caused by coupling to an environment. Such a phenomenon is theoretically analyzed by, for example, the Caldeira-Leggett model, which describes a quantum particle coupled to an environment represented as a collection of harmonic oscillators, and is also investigated by various experiments including cavity QED systems and quantum circuits. In this talk, I will briefly introduce these topics and may refer to our recent related work [Takeru Yokota, Kanta Masuki, Yuto Ashida, arXiv:2208.14107].

2023-01-27

## Quantum many-body physics with cold atoms

関野 裕太 (数理創造プログラム 特別研究員 / 理化学研究所 開拓研究本部 (CPR) 長瀧天体ビッグバン研究室 特別研究員)

Physical systems consisting of many quantum particles are ubiquitous in diverse fields of physics. Such systems called quantum many-body systems include solids surrounding us, superconductors used for quantum computations, and neutron matter in neutron stars. Understanding of such quantum many-body systems is, however, often challenging. Among various systems, cold atoms, which are atomic gases below 10^-6 kelvin, are ideal platforms for investigating quantum many-body physics because of their simplicity and high controllability. In this talk, I explain how studies of cold atoms have contributed to deepen our understanding of quantum many-body systems.

2023-01-20

## complexity in non-unitary dynamics of photons

Ken Mochizuki (Hamazaki Hakubi Lab.)

Sampling probability distributions of photons in optical networks can be hard for classical computers. Therefore, the boson sampling problem has been extensively explored from the viewpoint of the quantum supremacy. However, sampling photon distributions is not always hard and physical situations determine the hardness, which suggests that the computational complexity can be used to characterize quantum dynamics and quantum phases. I discuss the distinction between unitary and non-unitary dynamics from such a point of view, where the former and later respectively correspond to isolated and open quantum systems.

2023-01-13

## How can we describe the hierarchical structure of our Universe?

廣島 渚 (数理創造プログラム 客員研究員 / 富山大学 理学部 物理学科 助教)

Our Universe exhibits a highly hierarchical structure. Halos, which are gravitationally bounded objects of dark matter, are building blocks of such structures. Halos are distributed in more than 20 orders of magnitudes in the mass scale. For example, our Milky Way resides in a halo of which mass is about a trillion solar masses. Indications about the nature of dark matter could be obtained by studying the hierarchical structures of dark matter halos while it needs considerations about the schemes to cover a wide mass range. In this talk, I will introduce a simple framework to describe the structure based on analytical descriptions.

2022-12-23

## Physics Through the Looking Glass

プトゥラク・ジャイアクソナ (数理創造プログラム 特別研究員)

The terminology Carrollian has recently shown up in many fields of theoretical physics. In this short talk, I will explain what Carrollian means and why some physicists find it interesting.

2022-12-09

## Supernova signatures of neutrino mass ordering

マリア・マヌエラ・サエス (数理創造プログラム 特別研究員)

Supernovae events are one the most powerful cosmic sources of neutrinos with energies of several MeV. The emission of neutrinos and antineutrinos of all flavors carries away the gravitational binding energy of the compact remnant and drives its evolution from the hot initial to the cold final state. I will briefly describe how to detect these neutrinos from Earth, and how to use these data to address the neutrino mass ordering problem.

2022-12-02

## Quantum many body treatment of collective neutrino flavor oscillations

エルマル・ルラーバイ (数理創造プログラム 特別研究員)

Collective flavor oscillations occur in extreme environments like supernovae and binary neutron star mergers where many neutrinos are produced in a short period of time. I will describe the quantum many body treatment and how it effects the evolution.

2022-11-25

## Counting by Drawing

Michel Van Garrel (Assistant Professor, School of Mathematics, University of Birmingham, UK)

How many other circles can you find that meet each of the 3 circles in exactly 1 point? I will talk about fun questions like this one that are solved by drawing, and have been with mathematics all along.

2022-11-18

## Knot theory and its interactions with other fields

谷口 正樹 (数理創造プログラム 基礎科学特別研究員)

Knot theory is one of subjects in the field of topology. I'll exlain what are purposes of knot theory and its interactions with other fields.

2022-11-11

## New approach to spontaneous symmetry breaking by gradient flow

菊地 健吾 (数理創造プログラム 基礎科学特別研究員)

The spontaneous symmetry breaking (SSB) is one of the most important concept in the elementary particle physics. In this coffee meeting, I explain the fundamental content of the SSB briefly, and after that, I talk about our recently research, new approach to analyze the phase structure of SSB using the gradient flow method.

2022-11-04

## Waveform analysis of biological oscillatory models

儀保 伸吾 (数理創造プログラム 特別研究員)

In biological systems, many oscillatory phenomena emerge. For example, our sleep-wake rhythms are regulated by gene activity oscillation with a period of 24 hours. Time series of these biological oscillations are of various shapes. In this talk, I would like to talk about the effect of the waveform on period stability and synchronization.

2022-10-28

## A new B.O.A.T. in astrophysics

ドン・ウォレン (数理創造プログラム 研究員 / 理化学研究所 開拓研究本部 (CPR) 長瀧天体ビッグバン研究室 研究員)

On October 9 of this year, an extremely bright gamma-ray burst was detected — it has been called the "brightest of all time". I will briefly describe some of the ways this burst was so extraordinary, and what science we can do with such an unusual opportunity.

2022-10-21

## Geometirical chracteristics of a polymer chain: twist and writhe

横田 宏 (数理創造プログラム 特別研究員)

Polymer is a string-like molecules composed of molecular units. Many physical phenomena on polymers are described by using consecutive beads connected by springs (bead-spring model). Although this model is widely used, sometimes this model is not sufficient when the twist or writhe structures are considered. In this talk, I would like to introduce the kink structure (twist and writhe) and its mathematical and physical description. And then, I would like to talk about the computational treatment of twist and writhe.

2022-10-14

## Evolution and our daily rhythms

黒澤 元 (数理創造プログラム 専任研究員)

Imagine that you are in a room without information of time. 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. It is because we have daily rhythms in our body. Biological experiments have shown that not only humans but also other many species on the Earth have these daily rhythms. In this talk, unsolved problems about the rhythms, and some approaches from the point of view of dynamical system will be introduced.

2022-10-07

## Various approaches to the sign problem

松本 祥 (数理創造プログラム 特別研究員)

The Monte Carlo simulation is a powerful tool to study the non-perturbative aspect of quantum field theory. However, the Monte Carlo method is applicable to the system with a real action only. If the action is complex, it is difficult to handle the rapidly oscillating phase in the path integral, which is known as the sign problem. This problem prevents us from simulating various interesting systems, such as finite density QCD, topological theta term and real time evolution. In this talk, I introduce several approaches to overcome the sign problem and compare their features.

2022-09-30

## A primitive derivation of black hole entropy

横倉 祐貴 (数理創造プログラム 上級研究員)

Black holes have entropy. While a black hole occupies a three-dimensional spatial domain, the entropy is given by its two-dimensional surface area. In this sense, the entropy is holographic. However, its true origin is still unknown, and many researchers are studying it using various approaches. In this talk, I will provide an intuitive derivation of the entropy according to Bekenstein's first discussion in 1973. In particular, I will emphasize that it is the result of a combination of quantum theory and gravity. I will also give a brief review of the basics of physics so that people in other fields can enjoy how this mysterious formula appears.

2022-09-16

## Area law of entanglement entropy in quantum many body systems and its implication in tensor network calculation

ヤンタオ・ウー (数理創造プログラム 特別研究員)

In this coffee talk, I will explain the idea of entanglement entropy and how it has instructed people to construct a class of ground state wavefunction ansatz for quantum many-body systems. I will be pedagogical and explain the general construction in 1D and 2D. If time permits, I will explain how fermionic versions of them are realized.

2022-09-09

## What’s the value of reproductive value?

トーマス・ヒッチコック (数理創造プログラム 基礎科学特別研究員)

Populations are often heterogenous, composed of individuals of different sexes, ages, and condition. The way that genes flow between these different states across time can structure the ancestry of the population, and subsequently generate changes in allele frequency even in the absence of any other evolutionary forces. In this talk I discuss the concept of reproductive value, which provides a description of the expected long-term contribution each state makes to future populations. This tool allows us to aggregate the effects of different evolutionary forces across these different classes of individual, and thus better understand their relative importance. I briefly illustrate the usefulness of these concepts by discussing the evolution of senescence.

2022-09-02

## Coarse Notions of Curvature

クリスティ・コウジ・ケリー (数理創造プログラム 基礎科学特別研究員)

Curvature is a fundamental geometric notion with important applications in a variety of physical theories. Typically curvature is defined in smooth (differentiable) contexts but there has been much recent interest in synthetic characterisations of curvature in much rougher spaces than differentiable manifolds---including discrete spaces like networks. In this talk we aim to introduce some of the main coarse curvatures, particular in relation to optimal transport theory.

2022-08-26

## How to carry out a hadron experiment

冨田 夏希 (数理創造プログラム 客員研究員 / 京都大学 大学院理学研究科 附属サイエンス連携探索センター (SACRA) 特定助教)

I might be the only experimentalist in iTHEMS. I have been working for studying hadrons at SPring-8. Hadron experiments are unique in its large scale of equipment, time, man-power and budget. I would like to introduce how a hadron experimentalist carry out experiments.