今期の予定
日時：2025年4月9日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：古内 理人（量子多体）
題目：Theoretical study of frustrated quantum spin systems with exact diagonalization
概要：We study frustrated antiferromagnets using the exact diagonalization method. The type of frustration addressed in this research occurs in localized spin systems, such as those represented in half-filled Mott insulators. This frustration occurs when antiferromagnetic interactions construct networks with odd-gons as local structures, causing a variety of exotic phenomena. Extensive studies have been reported on the triangular lattice[1] and the kagome lattice[2] as typical frustrated systems. The Heisenberg model describes such localized spin systems.
Computational approaches such as the quantum Monte Carlo (QMC) method and the density matrix renormalization group (DMRG) method are effective methods to study the Heisenberg model. However, the QMC method suffers from the negative sign problem in frustrated systems, while DMRG faces challenges in interpreting results for systems with two or more spatial dimensions. To avoid these problems, we employ the exact diagonalization method, which, although restricted to systems with a small number of spins, can calculate frustrated system.
日時：2025年4月16日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：吉田 崇晴（東京理科大学）
題目：Proposal for experimental realization of quantum spin chains with quasiperiodic
interaction using Rydberg atoms
概要：Investigating the localization properties of interacting disordered systems plays a crucial role in understanding the fundamental origins of both the emergence and the breakdown of statistical mechanics in closed quantum systems. However, simulating such systems on classical computers is challenging due to their complexity. On the other hand, recent advances in experimental technology have made it possible to realize closed quantum systems with high controllability. In particular, Rydberg atoms [1] are an emerging platform for quantum simulation, as they offer the advantage of individual control over the spatial positions of atoms and the ability to tune their interactions.
In this talk, I will talk about my recent work proposing an experimental method to realize S=1/2 and S=1 quantum spin models with quasiperiodic interactions using Rydberg atoms [2]. I will also present numerical results for these models, confirming that they exhibit a many-body critical (MBC) regime [3], a newly reported localization phenomenon distinct from both ergodic and many-body localized phases.
[1] A. Browaeys, D. Barredo, and T. Lahaye, J. Phys. B At. Mol. Opt. Phys. 49, 152001 (2016).
[2] T. Yoshida, M. Kunimi, and T. Nikuni, arXiv:2409.08497 (2024).
[3] Y. Wang, C. Cheng, X.-J. Liu, and D. Yu, Phys. Rev. Lett. 126, 080602 (2021).
日時：2025年4月23日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：Jose Carlos Pelayo（量子多体）
題目：Ultracold Atoms: Applications in Metrology and Fundamental Properties of Quantum Droplets
概要：In recent decades, cold atoms have become indispensable tools for exploring fundamental physics and advancing quantum technologies. In this talk, I will delve into this broad landscape by presenting two topics from my PhD research, focusing on quantum metrology and quantum mixtures. In the first part of the talk, I will demonstrate how a Floquet-engineered bosonic system in a one-dimensional lattice can serve as a platform for quantum metrology. In the second part, I will turn to quantum mixtures involving a relatively new phase of matter known as quantum droplets, where I investigate the role of dimensionality in the formation of droplets.
日時：2025年4月30日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：田中 愛梨（物性理論）
題目：Dynamics of superfluid wake behind a plate-shaped obstacle
概要：In fluid mechanics, study of a flow that occurs behind an obstacle (wake) when a fluid passes through the obstacle is an important subject for understanding the behavior of fluids, and is therefore a subject of very active research. However, there has been less research on wakes in superfluids compared to classical fluids. The shapes of obstacles used in studies of the wake of superfluids are almost always assumed to be cylindrical, and the state of the wake around a plate-shaped obstacle has not yet been explored. In this study, we numerically investigate the fluid flow behind a plate-shaped obstacle by using the Gross–Pitaevskii equation, a model equation for the macroscopic wave function of a Bose-Einstein condensate. As a result, we are able to classify the wake state by changing the size and speed of the plate-shaped obstacle.
日時：2025年5月14日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：籔内 雄大（量子多体）
題目：Critical velocity in superfluid states of hardcore Bose-Hubbard model with long-range hopping
概要：Long-range interacting spin systems - such as Rydberg atom arrays, dipolar Bose gases, and trapped ions - have attracted significant attentions as a new platform for studying quantum many-body physics [1-5]. In particular, the spin-1/2 XY model with long-range interactions, which is realizable in Rydberg arrays [4] and trapped ions [5], can be effectively mapped onto a hard-core Bose-Hubbard model (HCBHM) with long-range hopping. Given that there is a superfluid state in the quantum phase diagram of the HCBHM with short-range interaction, it is interesting to study how the long-range nature of the hopping affects superfluidity.
In this study, we analyze the superfluid critical velocity by applying a mean-field theory [3] to the HCBHM with long-range hopping that decays algebraically as ~ 1/rb . We find that when the power of the algebraic decay b decrease, the critical velocity vanishes at b=3, which corresponds to the case of the XY model realized with Rydberg atom arrays. In contrast, when b increases toward the nearest-neighbor limit, the critical velocity converges to a finite value consistent with known results of the short-range interaction [3]. We also discuss how the critical lines of Mott-superfluid transition in the HCBHM with long-range hopping varies with b in this long-range interacting spin model.
[1] D. Peter, et al, Phys. Rev. Lett. 109, 025303 (2012)
[2] O. K. Diessel, et al, Phys. Rev. Research 5, 033038 (2023)
[3] I. Danshita and D. Yamamoto, Phys. Rev. A 82, 013645 (2010)
[4] C. Chen, et al, Nature 616, 691-695 (2023)
[5] N. Kotibhaskar, et al, Phys. Rev. Research 6, 033038 (2024)
日時：2025年5月21日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：久慈 浩輝（東京理科大学）
題目：Proposal for realizing quantum-spin systems on a two-dimensional square lattice with Dzyaloshinskii-Moriya interaction by the Floquet engineering using Rydberg atoms
概要：Quantum simulation is an effective method for solving complicated quantum many-body systems that are difficult to solve with a classical computer. Recently, quantum simulators using Rydberg atoms have attracted much attention, and highly controllable quantum spin models have been realized by using them [1]. So far, for the Hamiltonian of a spin system represented by the Rydberg state, pulses have been applied periodically in time and the XYZ model [2] and the XY interaction and the mono-axial Dzyaloshinskii-Moriya (DM) interaction [3] have been implemented by using Floquet engineering.
Indeed, the XYZ model has been realized experimentally [2]. We focus on the method proposed in the reference [3], which combines global and local pulses and is irradiated periodically. We extend this technique with an optical tweezers array of Rydberg atoms on a two-dimensional square lattice and use Floquet engineering to construct a Hamiltonian with the Heisenberg interaction and a DM interaction that depends on the bond direction. The advantage of this method is that the strength of the interaction can be controlled by changing the time interval of the pulses, making it possible to realize parameter regions where the DM interaction is stronger than the exchange interaction, which is difficult to achieve in real solids. This advantage is expected to lead to the study of skirmions in situations with strong quantum fluctuations. In this talk, I will present a method for constructing Hamiltonians with Heisenberg interactions and bond direction-dependent DM interactions in a system of Rydberg atoms on a two-dimensional square lattice with an optical tweezers array.
Reference:
[1] A. Browaeys and T. Lahaye, Nature Phys. 16, 132 (2020).
[2] S. Geier et al., Science 374, 1149 (2021).
[3] N. Nishad, et al., Phys. Rev. A 108, 053318 (2023).
日時：2025年5月28日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：Sidharth Rammohan（量子多体）
題目：Quantum dynamics of dipolar bosons in planar array using cluster mean-field theory
概要：Dipolar bosons in optical lattices have emerged as a powerful platform for the quantum simulation of condensed matter systems, owing to their high degree of tunability, particularly in controlling long-range interaction strengths. These systems have been extensively used to explore phase transitions in spin models and are now drawing increasing interest for studying exotic quantum phases such as Luttinger liquids. In a 2008 theoretical study, dipolar bosons confined to planar arrays were investigated as a potential experimental platform for realizing the sliding Luttinger liquid (SLL) phase—a quasi-two-dimensional state formed by weakly coupled, strongly interacting one-dimensional chains that retain Luttinger liquid behavior while exhibiting anisotropic, power-law correlations between chains [1]. Given the experimental challenges in observing the SLL phase directly, the primary goal of this work was to propose a viable platform based on dipolar bosons that enables its detection and characterization.
Despite the usual challenges associated with experimental advancements, experimental research has recently accelerated, and dipolar bosons can already be realized in optical lattices in laboratories. In 2023, Griener's group investigated the formation of dipolar quantum solids in a Hubbard quantum simulator [2]. Given the current status of the research field, we think that adopting a different approach, like a cluster mean-field method, to the problem will help to develop a computationally efficient solution and open the door to studying the real-time dynamics of the systems. We apply the cluster mean-field approach introduced in [3] to the system of dipolar array of 1D chains and investigate its feasibility by identifying the interaction regimes where the method yields reliable results.
While Ref. [3] focuses on phase transitions in a hardcore Bose-Hubbard model, the same cluster mean-field (CMF) method can be effectively extended to the case of dipolar bosons in planar arrays. In my talk, I will present this proposal, beginning with an introduction to the CMF approach from [3] and a review of recent progress in studying phase transitions in dipolar boson systems. I will then discuss our exact diagonalization results for the ground state of dipolar bosons in planar arrays. Following that, I will present results obtained using DMRG and compare them with the exact diagonalization data. Finally, I will share our preliminary findings on real-time dynamics, including a comparison between exact and CMF results. For this, we consider chains confined in harmonic traps and study the dynamical response when one chain is perturbed (or "kicked") in the presence of inter-chain long-range interactions. From this analysis, we aim to identify the range of interaction strengths over which the cluster mean-field (CMF) method remains accurate, by comparing its results with those from exact simulations.
References:
[1] C. Kollath et.al., PRL 100, 130403 (2008).
[2] Lin Su et.al., Nature 622 (2023).
[3] D. Yamamoto et.al., PRB 86, 054516 (2012).
日時：2025年6月4日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：金田 滉平（場の量子論・素粒子論研究室）
題目：Resurgence Theory and Exact WKB analysis
概要：Perturbation theory is an effective analytical method in quantum mechanics and quantum field theory, but when it is applied to higher orders, the perturbation series diverges. On the other hand, in the physics of strongly coupled domains, “non-perturbative effects” such as the tunneling effect become important and perturbation theory cannot be used. In this presentation, we will discuss two novel approaches to study the non-perturbative aspects of quantum mechanical systems: resurgence theory and Exact WKB analysis. In order to clarify the nontrivial relationship between divergent perturbative series and nonperturbative effects (Resurgence structure), we show that singularities appearing in the Borel transformations of the perturbative series correspond to nonperturbative effects. Based on this structure, we also show that the non-perturbative effects can be extracted from the perturbative series. Then, we show that exact quantization conditions can be obtained from the equation connecting Stokes domains using the Exact WKB analysis, which is an exact analysis by applying the Borel resummation method to the WKB solution of the Schrodinger equation.
日時：2025年6月11日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：鍛治 祐斗 （場の量子論・素粒子論研究室）
題目：Bosonic string theory
概要：When trying to unify general relativity and quantum mechanics, several problems arise. String theory purports to overcome these difficulties and to provide a consistent quantum theory of gravity. Quantum mechanics and other theories have considered a point particle as the smallest unit of matter, whereas string theory considers one-dimensional extended objects, called strings. In this theory, different particles correspond to different oscillation modes of the string. In this presentation, we introduce the simplest string theory, called the bosonic string. We also discuss the action of strings.
日時：2025年6月18日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：武上 響生（京都大学）
題目：Spin Green's function approach to the Kitaev model at Finite Temperature
概要：The Kitaev model, a quantum spin system defined on a two-dimensional honeycomb lattice, features an exactly solvable ground state with fractionalized Majorana fermion excitations[1]. One of the most compelling approaches to studying the Kitaev model is through the formalism of Majorana fermions, which maps spin degrees of freedom onto itinerant fermionic ones, thereby enabling an exact solution at zero temperature[1]. However, at finite temperatures, an exact solution becomes elusive even within the Majorana fermion framework.
In this study, we investigate the finite-temperature properties of the Kitaev model using the spin Green’s function formalism[2]. This approach provides a unified framework for calculating both thermodynamic and dynamical quantities. Moreover, the spin Green’s function formalism is well-suited for incorporating magnetic field effects and non-Kitaev interactions. We primarily compare our results with Majorana-based numerical simulations[3]. We first examine the ground-state energy by changing the anisotropy of the interaction. We then explore other physical properties to further investigate the model’s finite-temperature behavior.
[1] A. Kitaev, Ann. Phys. 321, 2 (2006).
[2] H.Takegami and T. Morinari Phys. Rev. B 111, 054413(2025).
[3] Y. Motome and J. Nasu, J. Phys. Soc. Jpn. 89, 012002 (2020).
日時：2025年6月25日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：植田 健太（量子多体）
題目：TBA
概要：TBA
日時：2025年7月2日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：田渕 陽大（場の量子論・素粒子論研究室）
題目：Vacuum Energy in Quantum Field Theory toward Understanding Cosmic Expansion
概要：In quantum field theory, the vacuum state (the lowest energy state) is a state that includes fluctuations involving the creation and annihilation of particles, and its energy is not zero. First, I show how this vacuum energy is mathematically derived in the case of a free field, and introduce the Casimir effect as an experimentally observable phenomenon, demonstrating that this energy is not merely a theoretical concept. In addition, I show that when the degrees of freedom of bosons and fermions are equal, the vacuum energy is completely canceled. Furthermore, although this vacuum energy is suggested to be related to dark energy that causes the accelerated expansion of the universe, there exists a significant discrepancy between the theoretical values based on the Standard Model and the observed results. This suggests the possibility that the vacuum energy is being canceled by some mechanism. Therefore, I introduce a supersymmetric Lagrangian in which the degrees of freedom of bosons and fermions are balanced, and show that the cancellation of vacuum energy is possible even when interactions are included. Finally, I mention the possibility that supersymmetry is broken in the real world, and suggest that the small remaining vacuum energy may serve as the driving force of cosmic expansion.
日時：2025年7月9日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：百合 巧（大阪大学）
題目：TBA
概要：TBA
日時：2025年7月16日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：相生 瑛怜奈（固体電子物理学研究室）
題目：TBA
概要：TBA
日時：2025年7月23日10:45-
教室： 31号館3階シミュレーション実験室 + Zoomでのオンライン配信
発表者：TBA
題目：TBA
概要：TBA