Coffee Meeting Log
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!)
Public2023-07-14
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.
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.
Public2023-04-14
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.
2023-03-10
Theoretical analysis of supernova neutrino data
Akira Harada (Special Postdoctoral Researcher, iTHEMS)
A supernova explosion is the explosive death of a massive star. Neutrino observation is important to examine the mechanism. However, a theoretical framework to analyze the observed neutrinos has not been established. In order to make a tractable theory, we constructed an analytic neutrino signal model based on the Lane-Emden equation and thermal neutrino emission. Besides, to bridge the theory and observation, I developed a code, "SPECIAL BLEND," to estimate the mass and radius of the central proto-neutron star by fitting the analytic model to the supernova neutrino observation with the Bayesian approach. Using SPECIAL BLEND, I analyzed the neutrinos from the supernova SN1987A and estimated the mass and radius of the central neutron star.
2023-03-03
Gravitational methods for condensed matter and a curious puzzle for superfluid effective field theory
Matteo Baggioli (Associate Professor, School of Physics and Astronomy, Shanghai Jiao Tong University, China)
At the beginning of my PhD in theoretical gravitational physics, my advisor gave me a copy of Kittel's book "Introduction to solid state theory" to study. I was confused, very confused! After almost ten years of research, I have fully embraced the power of the gauge-gravity duality as a tool to investigate open questions in different fields such as condensed matter or nuclear theory. After briefly reviewing the most salient features and successes of this program, I will share with you an interesting puzzle for the effective field theory of superfluids that I recently became aware of. Maybe, we can find a solution to it together!
Public2023-02-24
Physical and local observables in gravitational theory
Christophe Goeller (Humboldt Fellow, Ludwig-Maximilians-Universität München, Germany)
I will discuss a general formalism for the construction of dynamical reference frames and local and physical observables in gauge and gravitational theories and their interactions with the gauge charges and their associated field transformations.
Public2023-02-17
Can you hear the shape of a drum?
Christopher Bourne (Visiting Scientist, iTHEMS / Assistant Professor, Advanced Institute for Materials Research (AIMR), Tohoku University)
I will review what it means to 'hear the shape of a drum' and how it offers a glimpse into the interesting ways that analysis and geometry interact.
2023-02-10
Generative models and stochastic differential equations
Akinori Tanaka (Senior Research Scientist, iTHEMS / Senior Research Scientist, RIKEN Center for Advanced Intelligence Project (AIP))
Generative models, a.k.a statistical models, are widely accepted frameworks for generating “new” samples from given dataset. Recently, a class of generative models, called diffusion models, made great impact on not only researcher’s community but communities on SNS and industries also. I would like to introduce these developments within 15 min. on the blackboard.