One of the most fundamental challenges in theoretical physics is to uncover the physical properties of strongly-interacting quantum many-body systems. This problem is shared in both subatomic physics and condensed matter physics; e.g., to unveil ground state structures and dynamical aspects of quantum systems. However, it has been an unresolved issue to establish non-perturbative theoretical tools, which allows a reliable analytic approach to quantum many-body problems described by field theory. The Functional Renormalization Group (FRG) is proposed as one of the theoretical methods that facilitates the non-perturbative investigation of quantum many-body systems. The FRG has found applications in various fields of physics, ranging from particle and nuclear physics to condensed matter physics, leading to several unique achievements in each fields. The aim of this two-day workshop is to provide an overview of the recent applications and progress of FRG in various fields of physics, discuss future directions, and explore potential new collaborations that bridge different fields of physics.

A group of 22 undergraduate students in the GLEAP programme at Seoul National University will visit iTHEMS to hear short talks by our members, exchange one-on-one, and visit our facilities. I would like to encourage all available iTHEMS members to take part in this event which will be held in different spaces throughout the day, all at the RIKEN Wako campus Main Research Building:

15:50-16:10 at iTHEMS Common Room (#246-248)
Coffee break with iTHEMS members and SNU visitors

16:10-17:50 on 4th floor, room #435-437
Short talks by iTHEMS members

18:05-18:30 in 3rd floor common space
Short intro talks by SNU visitors

18:30-20:00 in 3rd floor common space
Free informal discussion between SNU visitors and iTHEMS members over some light food

[Note some slight changes in the times previously announced]

Type-I X-ray bursts are rapidly brightening phenomena triggered by the nuclear burning of light elements near the surface of accreting neutron stars. Most of the X-ray bursters show irregular behavior of light curves. However, some X-ray bursters are somehow quite regular, i.e., constant recurrence time and constant shaper of light curves, and are often called Clocked bursters, which are powerful sites to probe uncertainties of many model parameters such as accretion rate, the composition of accreted matter, reaction rates, neutron star structure, and temperature. In this study, we focus on the uncertainties of the equation of states, which determines the latter two properties. Based on our numerical models covering whole areas of neutron stars, we will present their impact on X-ray burst light curves. Furthermore, we will discuss the possibility of constraining the equation of states from Clocked bursters such as GS 1826-24 and 1RXS J180408.9-342058.

Planet formation starts from collisional growth of sub-micron-sized dust grains in a gas disk called a protoplanetary disk. They are expected to grow toward km-sized objects called planetesimals. The resulting planetesimals further coalesce by gravity and form planets. However, there are some barriers preventing planetesimal formation, which includes fast radial drift and collisional fragmentation of dust grains. To circumvent the barriers and to explain planetesimal formation, previous studies have proposed hydrodynamic instabilities of dusty-gas disks. The instabilities can cause dust clumping, and planetesimals form if the resulting clumps collapse self-gravitationally. We have been investigating the linear/nonlinear development of these dust-gas instabilities. We also found a new instability driven by collisional growth of dust, which can bridge a potential gap between the first dust growth and the later planetesimal formation via the previous instabilities. In this talk, I will introduce our work on the dust-driven instabilities and their impact on planetesimal formation.

Note: The format of this event has changed from hybrid to Zoom only. However, you will still be able to watch it on the screen in Room #359 of the Main Research Building.

(This is a joint seminar with iTHEMS Biology group.)
Recent advances in machine learning have enabled the extraction of intrinsic information from neural activities, a field known as neural decoding. In this presentation, I will introduce several machine learning methods recently developed for neural decoding analysis: 1) a method for visualizing subjective images in the human mind based on brain activity [1], 2) a supervised algorithm designed for predicting discrete ordinal variables [2], and 3) a fast classical algorithm algorithm inspired by quantum computation for approximating principal component analysis (PCA) and canonical correlation analysis (CCA), potentially allowing for the analysis of vast-dimensional neural data [3]. Following these presentations, I am eager to engage in discussions with participants at the RIKEN Quantum Seminar regarding potential collaborations.

(This is a joint seminar with iTHEMS Biology group.)
Bayesian mechanics is a framework that addresses dynamical systems that can be conceptualised as Bayesian inference. However, the elucidation of requisite generative models is required for empirical applications to realistic self-organising systems. This talk introduces that the Hamiltonian of generic dynamical systems constitutes a class of generative models, thus rendering their Helmholtz energy naturally equivalent to variational free energy under the identified generative model. The self-organisation that minimises the Helmholtz energy entails matching the system's Hamiltonian with that of the environment, leading to an ensuing emergence of their generalised synchrony. In short, these self-organising systems can be read as performing variational Bayesian inference of the interacting environment. These properties have been demonstrated with coupled oscillators, simulated and living neural networks, and quantum computers. This notion offers foundational characterisations and predictions regarding asymptotic properties of self-organising systems exchanging with the environment, providing insights into potential mechanisms underlying emergence of intelligence.

Title: Wave-like Dark Matter Search Using Qubits
Abstract:
The rapid controllability required for quantum computers makes the currently proposed quantum bit modalities also attractive as electromagnetic field sensors. One of the promising applications is wave-like dark matter searches, where the electric field converted from the coherent dark matter excites the qubits, leading to detectable signals [Phys. Rev. Lett. 131, 211001]. The quantum coherence between the qubits can be utilized to enhance the signal rate in a multi-qubit system. By designing an appropriate quantum circuit to entangle the qubits, it was found that the signal rate can scale proportionally to $n_q^2$, with $n_q$ being the number of sensor qubits, rather than linearly with $n_q$ [arXiv: 2311.10413]. In the seminar, we overview the theoretical framework of the search, elaborate on the signal-enhancing mechanism driven by quantum entanglement with specific examples of the quantum circuits, and discuss how the scheme can be implemented in the platform of future fault-tolerant quantum computers. We also provide the introduction of the experimental realization, and report the status of the experimental works carried out in UTokyo/ICEPP.

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. 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 interdisiplinary discussions and collaborationd were started. To share such progresses and extend collaborations, we organize the second workshop. 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 a half-day session composed of contributed talks is also planned.

This workshop is partially supported by iTHEMS-phys Study Group. This workshop is a part of the RIKEN Symposium Series.

The detailed information can be found in the workshop website.