Inverse problems are widely studied in various scientific fields, including mathematics, physics, and medical imaging (such as CT and MRI reconstructions). In this talk, I will present a novel method for solving inverse problems using the diffusion model, with an application to CT reconstruction. The diffusion model, which is a core component of recent image-generative AI, such as Stable Diffusion and DALL-E3, is capable of producing high-quality images with rich diversity. The imaging process in CT (i.e., CT reconstruction) is mathematically an inverse problem. When the radiation dose is reduced to minimize a patient's exposure, image quality deteriorates due to information loss, making the CT reconstruction problem highly ill-posed. In the proposed method, the diffusion model, trained with a large dataset of high-quality images, serves as a regularization technique to address the ill-posedness. Consequently, the proposed method reconstructs high-quality images from sparse (low-dose) CT data while preserving the patient's anatomical structures. We also compare the performance of the proposed method with those of other existing methods, and find that the proposed method outperforms the existing methods in terms of quantitative indices.

14:45-15:00 Teatime discussion
15:00-16:00 Talk by Dr. Ryusuke Hamazaki (RIKEN Hakubi Team Leader, Nonequilibrium Quantum Statistical Mechanics RIKEN Hakubi Research Team)
16:15-17:15 Talk by Dr. Teruaki Enoto (Associate Professor, Department of Physics, Division of Physics and Astronomy, Graduate School of Science, Kyoto University)
17:15-18:00 Discussion

This is a joint seminar hosted by the Mathematical Biology lab of Kyushu University where Jeffrey Fawcett (iTHEMS) and Cheng-Ruei Lee (National Taiwan University) will give talks about plant domestication. Both talks will be aimed at students and will include some basic introduction of the topic. The seminar will be held on-site at Kyushu University and also by zoom so please free to register and join.

Program:

Title: Domestication and dispersal process of common buckwheat
Speaker: Dr. Jeffrey Fawcett (RIKEN iTHEMS)
Abstract: Crop domestication has not only been an ideal model to study how selection drives evolution, it is also tightly linked to past human activity and contains useful information that can improve plant breeding. Common buckwheat (Fagopyrum esculentum), which is used to make “soba” noodles in Japan, was domesticated from a wild progenitor species distributed in Southwest China. We have been using whole-genome sequences of several hundred cultivated accessions from around the world and some wild progenitor accessions to study its process of domestication and subsequent dispersal throughout Eurasia including Japan. In this talk, I will first provide an overview of the domestication and dispersal process of common buckwheat based on archaeological findings. I will then discuss the domestication and dispersal process and adaptive evolution of common buckwheat based on results of our population genetic analyses [1].

Title: The domestication and expansion history of mung bean and adzuki bean: evidence from population genomics
Speaker: Prof. Cheng-Ruei Lee (National Taiwan University)
Abstract: Who domesticated the crops we eat? When and where? What happened after domestication? How did crops spread across the world? These are the questions that have fascinated archaeologists for a long time. Using modern genomics techniques, we aim to answer these questions from a different angle. In mung bean (Vigna radiata), we uncovered a unique route of post-domestication range expansion. This route cannot be explained simply by human activities alone; instead, it is highly associated with the natural climates across Asia. We showed how the trans-continental climatic variability affected the range expansion of a crop and further influenced local agricultural practices and the agronomic properties of mung bean varieties. In adzuki (Vigna angularis), we obtained solid evidence of its domestication in Japan, most likely by the Jomons. We identified and validated the causal mutations for the seed coat color change during domestication. Contrary to the common belief that important yield-ensuring phenotypes (e.g., loss of pod shattering) should be selected early during domestication, we revealed a unique order of domestication trait evolution that cannot be observed from archaeological records directly [2].

Please register via the form by November 22nd (Fri.). We will share the Zoom link with online participants on the morning of the event day.

Non-equilibrium phenomena are typically addressed through continuum descriptions based on local equilibrium and linear response theory, such as hydrodynamics. While effective, these approaches often overlook global characteristics. We propose Global Thermodynamics as a minimal-variable framework to describe weak non-equilibrium systems, focusing on two-phase coexistence under weak heat flux.

By introducing a unique global temperature and extending entropy to non-equilibrium systems with a non-additive term, the framework predicts phenomena like metastable state stabilization—beyond the scope of traditional heat conduction equations. This talk will outline the framework, its key predictions, and validation efforts through numerical simulations and experiments.

(This is a joint seminar with Informatin Theory Study Group.)

The Standard Model of Particle Physics, encapsulating the vast majority of our understanding of the fundamental nature of our Universe, is at its core a gauge theory. Much of the richness of its phenomenology can be traced back to the complicated interplay of its various gauged interactions. While massive theoretical and algorithmic developments in classical computing have allowed us to probe many of these aspects, there remain a plethora of open questions that do not seem amenable to these methods. With a fundamentally different computational strategy, quantum computers hold the potential to address these open questions. However, a long road lies ahead of us before this potential may be realized. In this talk, I discuss a key step on this journey: constructing lattice gauge Hamiltonians that can be efficiently simulated on digital quantum devices. In particular, I focus on recent work that develops a fully gauge fixed Hamiltonian for SU(2) without fermions. Not only is this formulation well-suited for "close to continuum" simulations, it is also significantly less non-local than might be initially expected.

Program:
16:00-16:05 Opening remarks --- Shinichiro Fujii (TRIP / iTHEMS)
16:05-16:35 Tensor network simulations of lattice gauge theory --- Yantao Wu (iTHEMS)
16:35-17:05 Quantum electrodynamics meets quantum chemistry: Theoretical foundations for polariton dynamics and control --- Himadri Pathak (iTHEMS / R-CCS)
17:05-17:35 Towards quantum advantage: imaginary Hamiltonian variational ansatz on the Schwinger model and the MaxCut problem --- Xiaoyang Wang (iTHEMS / R-CCS)
17:35-17:59 Researchers conducting collaborative research proposals (TRIP) --- 3 min.*8
      Takaaki Kuwahara (Kyoto Univ.)
      Maxime Medevielle (Univ. of Tokyo)
      Tanay Pathak (Kyoto Univ.)
      Koudai Sugimoto (Keio Univ.)
      Takayuki Suzuki (NICT)
      Hidetoshi Taya (Keio Univ.)
      Takahiro Terada (Nagoya Univ.)
      Tianchun Wang (Keio Univ.)
17:59-18:00 Closing remarks --- Seiji Yunoki (CPR / R-CCS / RQC)
18:10-19:30 Banquet at Cafeteria 1, Welfare and Conference Bldg (C61) 1F, RIKEN, Wako

We explore a scenario in which dark matter is a massive bosonic field, arising solely from quantum fluctuations generated during inflation. In this framework, dark matter exhibits primordial isocurvature perturbations with an amplitude of O(1) at small scales that are beyond the reach of current observations, such as those from the CMB and large-scale structure. Assuming a monochromatic initial power spectrum, we identify the viable parameter space defined by dark matter mass and the length scale of perturbations. A key prediction of this scenario is copious formation of subsolar dark matter halos at high redshifts.

Collective oscillations of neutrinos represent emergent nonlinear flavor evolution phenomena instigated by neutrino-neutrino interactions in astrophysical environments with sufficiently high neutrino densities. In this talk, after a brief introduction, it will be shown that neutrinos exhibit interesting entanglement behavior in simplified models of those oscillations. Attempts to study this behavior using classical and quantum computers will be described. An intriguing connection to the heavy-element nucleosynthesis, namely the possibility of neutrino entanglement driving a new kind of i-process nucleosynthesis, will be introduced,

A collision of relativistically accelerated large nuclei creates the hottest matter on Earth — quark-gluon plasma (QGP). The properties of QGP have been studied through comparisons of final-state particle distributions between theoretical models and experimental data.

To quantitatively constrain QGP properties, it is necessary to build Monte Carlo models that simulate the space-time evolution of the system throughout the entire collision process. This includes the initial matter production from the accelerated nuclei, the evolution of QGP, hadronisation, and the evolution of hadron gas. In this talk, I will first explain how theoretical models, based on relativistic hydrodynamics and hadronic transport, are conventionally built and how they successfully extract QGP properties.

Next, I will discuss a hot topic: the possibility of finding QGP in proton-proton collisions, based on results from a state-of-the-art model that includes both equilibrated and non-equilibrated systems. Also, I will introduce a novel Monte Carlo initial state model based on perturbative QCD minijet production supplemented with a saturation picture. This Monte-Carlo EKRT model is one of the first initial state models for hydrodynamics to describe initial particle production from small to large momentum within a single framework, where total energy-momentum and charge conservations are imposed.

When simulating fermionic quantum systems, non-perturbative Monte Carlo techniques are often the most efficient approach known to date. However, beyond half filling they suffer from the so-called sign problem, i.e. negative "probabilities", so that stochastic sampling becomes infeasible. Recently, considerable progress has been made in alleviating the sign problem by deforming the integration contour of the path integral into the complex plane and applying machine learning to find near-optimal alternative contours. In this talk, I am going to present a particularly successful architecture, based on complex-valued affine coupling layers. Furthermore, I will demonstrate how insight gained from the trained network can be used for simpler analytic approaches.

The quantisation of gravity is widely believed to result in gravitons - particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here we show that signatures of single gravitons can be observed in laboratory experiments. We show that stimulated and spontaneous single graviton processes can become relevant for massive quantum acoustic resonators and that stimulated absorption can be resolved through optomechanical read-out of single phonons of a multi-mode bar resonator. We analyse the feasibility of observing a signal from the inspiral, merger and post-merger phase of a compact binary inspiral. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photoelectric effect for photons, such signatures can provide the first experimental evidence of the quantisation of gravity.

[1] G. Tobar, S. K. Manikandan, T. Beitel, and I. Pikovski, Nature Communications 15, 7229.
[2] G. Tobar, Igor Pikovski ,Michael E. Tobar, arXiv:2406.16898 (2024).

Abstract: Nontrivial topological defects such as knotted solitons called hopfions have been observed in a variety of materials including chiral magnets, nematic liquid crystals and even in ferroelectrics as well as studied in other physical contexts such as Bose-Einstein condensates. These topological entities can be modeled using the relevant physical variable, e.g., magnetization, polarization or the director field. Specifically, we find exact static soliton solutions for the unit spin vector field of an inhomogeneous, anisotropic three-dimensional (3D) Heisenberg ferromagnet and calculate the corresponding Hopf invariant H analytically and obtain an integer, demonstrating that these solitons are indeed hopfions [1]. H is a product of two integers, the first being the usual winding number of a skyrmion in two dimensions, while the second encodes the periodicity in the third dimension. We also study the underlying geometry of H, by mapping the 3D unit vector field to tangent vectors of three appropriately defined space curves. Our analysis shows that a certain intrinsic twist is necessary to yield a nontrivial topological invariant: linking number [2]. Finally, we focus on the formation energy of hopfions to study their properties for potential applications.

Short bio: Avadh Saxena is former Group Leader of the Condensed Matter and Complex Systems group (T-4) at Los Alamos National Lab, New Mexico, USA where he has been since 1990. He is also an affiliate of the Center for Nonlinear Studies at Los Alamos. His main research interests include phase transitions, optical, electronic, vibrational, transport and magnetic properties of functional materials, device physics, soft condensed matter, non-Hermitian quantum mechanics, geometry, topology and nonlinear phenomena & materials harboring topological defects such as solitons, polarons, excitons, breathers, skyrmions and hopfions. He recently completed a book on “Phase Transitions from a Materials Perspective” (Cambridge University Press, 2024). He is an Affiliate Professor at the Royal Institute of Technology (KTH), Stockholm, Sweden and holds adjunct professor positions at the University of Barcelona, Spain, University of Crete, Greece, Virginia Tech and the University of Arizona, Tucson. He is Scientific Advisor to National Institute for Materials Science (NIMS), Tsukuba, Japan. He is a Fellow of Los Alamos National Lab, a Fellow of the American Physical Society (APS), a Fellow of the Japan Society for the Promotion of Science (JSPS) and a member of the Sigma Xi Scientific Research Society, APS and American Ceramic Society (ACerS).

Emergence of space-time is a key concept in matrix models as a nonperturbative formulation of string theory. In this lecture, starting with a brief introduction to nonperturbative effects in string theory, I will review various aspects of emergence of space-time in matrix models. The topics I discuss include dynamical triangulation, double scaling limit, eigenvalue instanton, large-N reduction, T-duality for D-brane effective theories (orbifolding), noncommutative geometry and covariant derivative interpretation. Finally, I will introduce the type IIB matrix model.

(This is the 7th Intensive Lectures by Quantum Gravity Gatherings in iTHEMS. )

Program
December 17
10.15~10.30 Registration and Coffee
10.30~12.00 Lecture 1
12.00~13.30 Lunch
13.30~15.00 Lecture 2
15.00~16.00 Coffee break
16.00~17.00 Lecture 3
17.30~19.30 Banquet

December 18
10.15~11.45 Lecture 4
11.45~13.30 Lunch
13.30~15.00 Lecture 5
15.00~16.00 Coffee break
16.00~17.00 Lecture 6

December 19
10.15~11.45 Lecture 7
11.45~13.30 Lunch
13.30~15.00 Lecture 8
15.00~16.00 Coffee break
16.00~17.00 Lecture 9

Normalizing Flows (NFs) are a class of deep generative models that have recently been proposed as efficient samplers for Lattice Field Theory. Although NFs have demonstrated impressive performance in toy models, their scalability to larger lattice volumes remains a significant challenge, limiting their application to state-of-the-art problems. A promising approach to overcoming these scaling limitations involves combining NFs with non-equilibrium Markov Chain Monte Carlo (NEMCMC) algorithms, resulting in Stochastic Normalizing Flows (SNFs). SNFs harness the scalability of MCMC samplers while preserving the expressiveness of NFs. In this seminar, I will introduce the concepts of NEMCMC and NFs, demonstrate their combination into SNFs, and outline their connections with non-equilibrium thermodynamics. I will conclude by discussing key aspects of SNFs through their application to Effective String Theory, SU(3) gauge theory, and conformal field theory.

Neutron stars - the densest stars in the Universe - cool down mainly by loss of neutrinos, emitted from the stars' interior due to particle reactions. By comparing cooling models with observed surface temperature or luminosity, one can probe the properties of high-density matter, such as what kind of particles and states exist inside neutron stars. In this presentation, I will first review cooling theory, focusing on the neutrino cooling processes. In particular, we focus on the equation of state (EOS) uncertainties, which significantly affect cooling curves. We discuss aspects such as the effect of including hyperons in our EOS. Using the updated cooling code, C-HERES, we calculate cooling curves with different EOS. Finally, we present the future prospects for this study.

Research on planet formation involves various approaches, including explorations of small solar system bodies, observations of protoplanetary disks, dust experiments, simulations, and theoretical studies. One of the primary objectives in this field is to develop a comprehensive theory that explains how kilometer-sized planetesimals form from micrometer-sized dust grains, drawing upon findings from these diverse research methods.

This workshop will focus on the concept of pebbles, which play a crucial role in the planet formation process. Pebbles — typically defined as solids ranging from millimeter to centimeter in size — are intermediate building blocks in planet formation, though their definition varies depending on the context. Assuming pebbles has led to theoretical advances in mechanisms such as streaming instability and pebble accretion, which promote the formation and growth of planetesimals. Additionally, pebbles have been linked to barriers against dust growth, such as the bouncing barrier. Furthermore, observations of protoplanetary disks have revealed the size distribution and porosity of solids, while the strength and thermal conductivity of comets obtained by the Rosetta mission suggest the accumulation of pebbles due to disk instabilities. However, inconsistencies have been pointed out between pebble formation and theories of dust growth.

This workshop aims to revisit and refine our understanding of solid materials implicated in planet formation, particularly in light of findings from solar system explorations and protoplanetary disk observations. We aim to reevaluate the definition and role of pebbles in the broader context of planet formation, with a special focus on the current challenges and open questions in the field. The workshop will include discussions of experiments and simulations of dust growth and collisions, and planetesimal formation mechanisms such as streaming instability. The workshop features keynote talks from the perspectives of explorations, observations, experiments, simulations, and theories, and we also call for presentations on related topics.

The density functional theory (DFT) is one of the powerful methods to solve quantum many-body problems, which, in principle, gives the exact energy and density of the ground state. The accuracy of DFT is, in practice, determined by the accuracy of an energy density functional (EDF) since the exact EDF is still unknown. Currently, DFT has been used in many communities, including nuclear physics, quantum chemistry, and condensed matter physics, while the fundamental study of DFT, such as the first principle derivations of an accurate EDF and methods to calculate many observables from obtained densities and excited states, is still ongoing. However, there has been little opportunity to have interdisciplinary communication.

On December 2022, we had the first workshop on this series (DFT2022) at Yukawa Institute for Theoretical Physics, Kyoto University, and several interdisciplinary discussions and collaborations were started. On February 2024, we had the second workshop on this series (DFT2024) at RIKEN Kobe Campus, and more stimulated discussion occured. To keep and extend collaborations, we organize the third workshop. Since the third workshop, we extend the scope of the workshop to the development and application of DFT as well. In this workshop, the current status and issues of each discipline will be shared towards solving these problems by meeting together among researchers in mathematics, nuclear physics, quantum chemistry, and condensed matter physics.

This workshop mainly comprises lectures/seminars on cutting-edge topics and discussion, while sessions composed of contributed talks are also planned.