Seminar
603 events
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Seminar
Classical and Quantum Chaos
May 12 (Thu) at 16:00 - 17:00, 2022
Akira Shudo (Professor, Department of Physics, Graduate School of Science, Tokyo Metropolitan University)
Classical and quantum mechanics in multi-dimensions are qualitatively different from those in one-dimension since they are no more integrable in general and chaos appears in the dynamics. This brings a great deal of complexity or even richness both in classical and quantum dynamics. Especially in generic nonintegrable systems which are neither completely integrable nor fully chaotic, phase space becomes a mixture of regular and chaotic components. Such an aspect is a source of inexhaustible questions not only in the past but in the future. We here overview classical and quantum chaos in Hamiltonian systems.
Venue: via Zoom
Event Official Language: English
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Introduction to Topological Insulators: Topological Superconductors and Quantum Computing
May 9 (Mon) at 14:00 - 15:30, 2022
Ching-Kai Chiu (Senior Research Scientist, iTHEMS)
Venue: via Zoom
Event Official Language: English
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Diversity of Asgardarchaota and Theoretical verification of the endosymbiotic theory
April 28 (Thu) at 10:00 - 11:00, 2022
Daiki Kumakura (Ph.D. Student, Graduate School of Life Science, Hokkaido University)
How did intracellular symbiosis occur and give rise to eukaryotic ancestor? This question has been considered to the two theories as three-domain theory and eocyte theory. Here I present asgard archaea, the archaeon closest to eukaryotes. Asgard archaea is an archaeon found at a deep-sea sampling site called Loki's castle at between Greenland and Norway. So all the closely related species are named after Norse mythology (Loki-, Thor-, Odin-, Heimdall-, etc.). Unlike other archaea, asgard archaea has many eukaryotic-specific proteins and is considered to be the closest to eukaryotes. In 2020, one of the asgard archaea species was finally successfully cultured. This archaeon was cultured and found to take on a branch-like structure. It is then hypothesized that intracellular symbiosis between this archaeon and the ancestor of mitochondria resulted in the ancestor of today's eukaryotic cells. In this talk, I would like to discuss with you the explanation of how we arrived at this hypothesis and how to construct a mathematical model.
Venue: via Zoom
Event Official Language: English
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Seminar
Recurrence theorems for topological Markov chains
April 22 (Fri) at 17:00 - 19:00, 2022
Cédric Ho Thanh (Postdoctoral Researcher, Prediction Science Laboratory, RIKEN Cluster for Pioneering Research (CPR))
Recurrence theorems place conditions under which probabilistic systems, specifically Markov chains, are expected to visit certain states infinitely often. For example, a printer with its many moving parts and the random requests it receives, may be described as a probabilistic system, and recurrence of the "ready to print" state is desirable. Recurrence theorems in the case of finite Markov chains are widely known. In this talk, we are interested in generalization to the infinitary setting. As it turns out, some care has to be put in the definition of infinite Markov chains. Rather than simply infinite, the introduct topological Markov chains, and show how standard constructions can be naturally extended to thisframework: path spaces, cylinder sets, as well as the semantic of LTL and PCTL. With all these tools in hand, we finally state our recurrence theorems. This is work in progress in collaboration with Natsuki Urabe and Ichiro Hasuo. This seminar is hold in a hybrid style. If you want attend the seminar onsite, please contact to Keita Mikami.
Venue: Hybrid Format (Common Room 246-248 and Zoom) (Main Venue)
Event Official Language: English
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Seminar
Coherent emission from 3D relativistic shocks
April 22 (Fri) at 14:00 - 15:00, 2022
Masanori Iwamoto (Kyushu University)
The origin of fast radio bursts (FRBs; Lorimer et al. 2007) is one of the unsolved problems in astrophysics. Many observations of FRBs indicate that FRBs must be coherent emission in the sense that coherently moving electrons radiate electromagnetic waves. In relativistic shocks, it is well known that coherent electromagnetic waves are excited by synchrotron maser instability (SMI) in the shock transition (Hoshino & Arons 1991). The SMI is also known as the emission mechanism of coherent radio sources such as auroral kilometric radiation at Earth and Jovian decametric radiation. Recently, some models of fast radio burst based on the coherent emission from relativistic shock via the SMI have been proposed (e.g., Lyubarsky 2014; Beloborodov 2017; Plotnikov & Sironi 2019; Metzger et al. 2019) and the SMI in the context of relativistic shocks attracts more attention from astrophysics. In this study, by performing the world’s first three-dimensional (3D) particle-in-cell (PIC) simulation of relativistic shocks, we will demonstrate that large-amplitude electromagnetic waves are indeed excited by the SMI even in 3D and that the wave amplitude is significantly amplified and comparable to that in pair plasmas due to a positive feedback process associated with ion-electron coupling. Based on the simulation results, we will discuss the applicability of the SMI for FRBs in this talk.
Venue: via Zoom
Event Official Language: English
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Seminar
Neurons are potential statisticians
April 21 (Thu) at 10:00 - 11:00, 2022
Takuya Isomura (Unit Leader, Brain Intelligence Theory Unit, RIKEN Center for Brain Science (CBS))
Humans and animals can predict what will happen in the future and act appropriately by inferring how the sensory inputs were generated from underlying hidden causes. The free-energy principle is a theory of the brain that can explain how these processes occur in a unified way. However, how the fundamental units of the brain, such as the neurons and synapses, implement this principle has yet to be fully established. Here, we have mathematically shown that neural networks that minimise a cost function implicitly follow the free-energy principle and actively perform statistical inference. We have reconstructed a biologically plausible cost function for neural networks based on the equation of neural activity and shown that the reconstructed cost function is identical to variational free energy, which is the cost function of the free-energy principle. This equivalence speaks to the free-energy principle as a universal characterisation of neural networks, implying that even at the level of the neurons and synapses, the neural networks can autonomously infer the underlying causes from the observed data, just as a statistician would. The proposed theory will advance our understanding of the neuronal basis of the free-energy principle, leading to future applications in the early diagnosis and treatment of psychiatric disorders, and in the development of brain-inspired artificial intelligence that can learn like humans.
Venue: via Zoom
Event Official Language: English
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Seminar
Light-matter control of quantum materials: From light-induced superconductivity to cavity materials
April 20 (Wed) at 15:30 - 17:00, 2022
Michael Sentef (Emmy Noether Research Group Leader, Max Planck Institute for the Structure and Dynamics of Matter, Germany)
In this talk I will discuss recent progress in controlling and inducing materials properties with light [1]. Specifically I will discuss recent experiments showing light-induced superconductivity through phonon driving in an organic kappa salt [2] and its possible theoretical explanation via dynamical Hubbard U [3]. I will then highlight some recent theoretical and experimental progress in cavity quantum materials [4], where the classical laser as a driving field of light-induced properties is replaced by quantum fluctuations of light in confined geometries. Ideas and open questions for future work will be outlined.
Venue: via Zoom
Event Official Language: English
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Coarse-grained molecular dynamics simulation via Langevin simulation
April 14 (Thu) at 10:00 - 11:00, 2022
Hiroshi Yokota (Postdoctoral Researcher, iTHEMS)
In the cell biology or biophysics, many mechanical properties of proteins or DNA are discussed. In order to consider the dynamics, coarse-grained molecular dynamics simulation (Langevin simulation) is useful. In this seminar, I will give you the introductory and methodology talk about the Langevin simulation.
Venue: via Zoom
Event Official Language: English
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Seminar
iTHEMS - Kyoto University Joint Seminar: Single-trajectory map equation
April 1 (Fri) at 8:30 - 10:00, 2022
Tatsuro Kawamoto (Researcher, Artificial Intelligence Research Center, National Institute of Advanced Industrial Science and Technology (AIST))
This seminar is a joint seminar of Blockchain research group in Kyoto University and EcoP WG in iTHEMS.
Venue: via Zoom
Event Official Language: English
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Journal Club: Phase separation in a many-component system with random interactions
March 31 (Thu) at 10:00 - 11:00, 2022
Kyosuke Adachi (Special Postdoctoral Researcher, Nonequilibrium Physics of Living Matter RIKEN Hakubi Research Team, RIKEN Center for Biosystems Dynamics Research (BDR))
Several kinds of protein condensates have been observed in living cells, and the liquid-liquid phase separation is regarded as a basic mechanism of the condensate formation. However, given that there are thousands of protein species in a cell, it is not clear how the number and the composition of distinct condensates are controlled. One of the physics approaches to this problem is considering a model of many components with random interactions. In this Journal Club, I will introduce a recent paper [1] that applies random-matrix theory to the phase separation dynamics.
Venue: via Zoom
Event Official Language: English
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Seminar
Hydrodynamic theory of electron and spin transport
March 30 (Wed) at 13:30 - 15:00, 2022
Gen Tatara (Team Leader, Spin Physics Theory Research Team, RIKEN Center for Emergent Matter Science (CEMS))
Electron and spin transports in metals are theoretically studied from a hydrodynamic viewpoint by calculating momentum flux density as a linear response to an applied electric field. Dissipative (ohmic) fluid regime is considered. An angular momentum generation in chiral (Weyl) system and spin motive force (voltage generation) by magnetization-vorticity coupling in anomalous Hall system are discussed. The spin Hall effect is argued from the viewpoint of a spin-vorticity coupling.
Venue: via Zoom
Event Official Language: English
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Seminar
Local and global topology for Dirac points with multi-helicoid surface states
March 24 (Thu) at 17:00 - 18:15, 2022
Tiantian Zhang (Specially Appointed Assistant Professor, School of Science, Tokyo Institute of Technology)
Though topological invariants defined for topological semimetals are usually local ones, they also have a global nature. For example, the Z type local monopole charge C for Weyl points, has a global nature, telling us its influence to the rest of the Brillouin zone, giving rise to bulk-surface correspondence associated with helical surface states. In Dirac systems, helical surface states are not guaranteed due to C=0. However, a new bulk-surface correspondence associated with double/quad-helicoid surface states (DHSSs/QHSSs) can be obtained for Dirac points with the protection of a Z2 type monopole charge Q, which is defined in terms of the time-reversal (T)-glide (G) symmetry (TG)2= -1. Here we study the topology of Q for Z2 Dirac points and establish its bulk-surface correspondence with strict proofs. We find that Q is equivalent to the G-protected Z2 invariant v mathematically and physically in Z2 Dirac systems. This result is counterintuitive, since v is always trivial in T-preserving gapped systems, and was thought to be ill-defined in gapless systems. We offer a gauge-invariant formula for Q, which is associated with DHSSs in both the spinless and spinful systems with single G. Q is formulated in a simpler form in spinless systems with two vertical G, associated with QHSSs, which is also entangled with filling-enforced topological band insulators in three space groups when a T-breaking perturbation is introduced. Since Q is ill-defined in spinful systems with two vertical G, QHSSs will not be held. Material candidate Li2B4O7 together with a list of possible space groups preserving QHSSs are also proposed for demonstration on our theory and further studies. *Detailed information about the seminar refer to the email.
Venue: via Zoom
Event Official Language: English
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Criticality in stochastic SIR model for infectious diseases based on path-integral approach
March 24 (Thu) at 10:00 - 11:00, 2022
Shigehiro Yasui (Assistant Professor, Center of Medical Information Science, Kochi Medical School)
The susceptible-infected-removed (SIR) model provides us with a basic scheme for the analysis of the epidemic infectious diseases such as the COVID-19. In this presentation, we focus on the stochastic SIR model which describes the stochastic time-evolutions of the population sizes for the susceptible, infected, and removed individuals. We consider the master equation (Kolmogorov forward equation) for the infection transmission and recovery processes (SI->II and I->R), and transform it into the Hamiltonian formalism with the Fock space a la quantum physics. According to the Doi-Peliti prescription, furthermore, we introduce the path-integral formalism similar to the quantum field theory, and perform the perturbative and non-perturbative calculations for the time-evolution of the susceptible, infected, and removed populations. We find that the critical value Rc of the basic reproduction number, which determines the spreading or the convergence of the infectious diseases, can be modified by the stochastic effects in comparison to the Rc in the conventional deterministic SIR model.
Venue: via Zoom
Event Official Language: English
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Seminar
Explore the possibility to control hurricanes
March 18 (Fri) at 16:00 - 18:00, 2022
Lin Li (Postdoctoral Researcher, Prediction Science Laboratory, RIKEN Cluster for Pioneering Research (CPR))
Hurricanes, also known as tropical cyclones and typhoons, are the biggest and the most devastating storms on Earth. In this seminar, I will talk about the possibility to control hurricanes with existing human capability. Energetically speaking, controlling hurricanes is a very challenging task due to a large gap: hurricanes are gigantic heat engines with a power of around 1014 Watt, while the most powerful manmade engines have the power of only 108 Watt. This six-order-magnitude gap is the major obstacle toward using existing engines to control hurricanes. To fill in this gap, we propose to utilize the chaotic nature of hurricanes, namely, the sensitivity of a chaotic system to its initial condition, to control hurricanes. In this presentation, I will first review the basics of hurricanes and existing chaos control methods, and then present my thoughts on hurricane control and preliminary results I acquired since joining Prediction Science Laboratory. Future directions on using reinforcement learning to control hurricanes will also be discussed. Since it is a very challenging task, I welcome any discussions, questions, and comments. I hope we can make the hurricane-risk-free future come earlier.
Venue: via Zoom
Event Official Language: English
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Phylogenomics revealed one of the problems for phylogeny –The monophyly of Archaeplastida including land plant-
March 17 (Thu) at 10:00 - 11:00, 2022
Euki Yazaki (Postdoctoral Researcher, iTHEMS)
There are many problems between large eukaryotic lineages. One of these is the monophyly of Archaeplastida to which land plants and other photosynthetic organisms belong. Although it has been believed that the Archaeplastida are monophyletic because they share common chloroplast structures, several large-scale molecular phylogenetic analyses have failed to reproduce this phylogenetic relationship. In this study, by enhancing the taxon sampling of the data set, the monophyly of Archaeplastida was successfully reconstructed, showing that the taxa critical for the reconstruction are present. Through detailed molecular phylogenetic and statistical analyses, it was estimated that the lack of monophyly ofArchaeplastida is due to the specific evolutionary signals of certain taxa.
Venue: via Zoom
Event Official Language: English
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Seminar
Extracting rules from trained machine learning models with applications in Bioinformatics
March 11 (Fri) at 16:00 - 18:00, 2022
Pengyu Liu (Postdoctoral Researcher, Medical Data Mathematical Reasoning Team, RIKEN Information R&D and Strategy Headquarters (R-IH))
Recently, Machine learning methods have achieved great success in various areas. However, some machine learning-based models are not explainable (e.g., Artificial Neural Networks), which may affect the massive applications in medical fields. In this talk, we first introduce two approaches that extract rules from trained neural networks. The first one leads to an algorithm that extracts rules in the form of Boolean functions. The second one extracts probabilistic rules representing relations between inputs and the output. We demonstrate the effectiveness of these two approaches by computational experiments. Then we consider applying an explainable machine learning model to predict human Dicer cleavage sites. Human Dicer is an enzyme that cleaves pre-miRNAs into miRNAs. We develop an accurate and explainable predictor for the human Dicer cleavage site -- ReCGBM. Computational experiments show that ReCGBM achieves the best performance compared with several existing methods. Further, we find that features close to the center of pre-miRNA are more important for the prediction.
Venue: via Zoom
Event Official Language: English
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Seminar
Toward modeling complete supernova neutrino emissions
March 11 (Fri) at 16:00 - 17:00, 2022
Yudai Suwa (Associate Professor, Department of Earth Science and Astronomy, Graduate School of Arts and Sciences, The University of Tokyo / Affiliate Associate Professor, Yukawa Institute for Theoretical Physics, Kyoto University)
Neutrinos are guaranteed observable from the next Galactic supernova (SN). Optical lights and gravitational waves are also observable but can be difficult to observe if SN location in the galaxy and the explosion details are unsuitable. The key to the next coming SN observation will be understanding various physical quantities using neutrinos first and then connecting them to other signals. In particular, understanding neutrinos in the late time (> 1 sec after the onset of explosion) is essential, since physics in this time scale has much smaller uncertainties than that of the early time. We should construct a simple and understandable neutrino model based on the late-time emissions. It allows us to tackle the physics in the early phase like the explosion mechanism. In this talk, I will discuss the following topics: 1) how to model the complete neutrino emissions from the very early phase up to the last observable event. 2) what physical quantities (e.g., mass and radius of neutron stars) can be extracted from observations using large statistical neutrinos as physics probes. 3) how to use these extracted physical quantities to link with the explosion mechanism of SN and multi-messenger observations.
Venue: via Zoom
Event Official Language: English
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Seminar
Nonperturbative cavity/waveguide quantum electrodynamics and dissipative quantum phase transition
March 10 (Thu) at 13:30 - 15:00, 2022
Yuto Ashida (Associate Professor, Graduate School of Science, The University of Tokyo)
Strong coupling between matter and quantized electromagnetic modes in cavity or waveguide may offer yet another approach of controlling equilibrium phases or dynamics of many-body systems. Recent developments have realized such strong light-matter interaction in genuinely quantum and nonperturbative regimes, where conventional approximate theoretical methods cannot be applied in general. I will talk about how one can analyze strongly coupled quantum light-matter systems at arbitrary interaction strengths on the basis of an asymptotically disentangling unitary transformation [1,2]. I discuss its application to construction of tight-binding Hamiltonians, dynamics of bound states in the continuum, and revisiting dissipative quantum phase transition in resistively shunted Josephson junctions [3].
Venue: via Zoom
Event Official Language: English
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Independent regulation of multiple checkpoints in cell-cycle network system -Biological function originated in the law of localization-
March 10 (Thu) at 10:00 - 11:00, 2022
Atsushi Mochizuki (Professor, Institute for Frontier Life and Medical Sciences, Kyoto University)
In cell cycle, G1-S and G2-M checkpoints are regulated by different protein complexes, Cdc2-Cdc13 and Cdc2-Cig2, respectively. For a normal mitosis, activity of two complexes should rise specifically at different timing. However, the complex formations share common species of proteins and activation reactions conform a complicated network. We study how independent regulation of two checkpoints is realized in the network system by “structural sensitivity analysis”, which was previously established by us. The analyses clarified that activities of two complexes are regulated by disjoint sets of reaction parameters in the system. A series of non-trivial behaviors are generated by “buffering structures with an intersection”, which can generally appear in chemical reaction network including complex formation.
Venue: via Zoom
Event Official Language: English
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Seminar
Introduction to stability conditions 2
March 9 (Wed) at 16:00 - 17:30, 2022
Naoki Koseki (Postdoctoral Research Associate, School of Mathematics, University of Edinburgh, UK)
In 2002, Bridgeland defined the notion of stability conditions on a triangulated category, motivated by string theory and mirror symmetry. Since then, Bridgeland stability conditions have been found very useful not only in Mathematical Physics, but also in various areas of Pure Mathematics. In the first part, I will review basic background and open problems in the theory of Bridgeland stability conditions. In the second part, I will explain recent developments of the theory, especially its applications to algebraic geometry.
Venue: via Zoom
Event Official Language: English
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