Kindai QPT Seminar

今期の予定

日時：2025年9月24日15:00-

教室：31号館3階シミュレーション実験室

発表者：小林 麟太郎（量子多体）

題目：量子誤り訂正の理論と量子コンピュータ実機での実装例の紹介

概要：量子コンピュータは素因数分解や量子シミュレーションなどの古典計算機では計算が困難あるいは不可能な特定の問題を解く可能性を秘めており、その実現に向けた近年のハードウェアの技術発展により世界的に注目を集めている。しかし、量子状態は外部環境のノイズや操作・観測の不正確性により意図しない変化を起こすため、信頼できる量子計算の実現には量子誤り訂正（QEC）が不可欠である。本発表では、まずQEC の基礎として古典誤り訂正との類似性や3-qubit codeによる簡単な例を通してどのように量子情報を冗長化してエラーを検出・訂正するかを説明する[1]。次に、実用的な量子誤り訂正符号(QECC)として注目されるSurface codeを取り上げ、その構造やエラーの検出・訂正の仕組みを説明する[2]。最後に、Google Quantum AI が超伝導量子プロセッサ「Willow」によりSurface codeを実装した実験論文[3,4]について紹介する。論文[4]では、最大101個のqubit を用いてSurface code を動作させ、qubit 数の増加によって計算の正確性が向上することを実証するとともに、その性能の限界を示唆する結果も得られている。

[1]中田 芳史, 『量子情報理論 ―情報から物理現象の理解まで―』, 朝倉書店, 東京 (2024). 

[2]A. Pesah, “An interactive introduction to the surface code,” Blog (2023), https://arthurpesah.me/blog/2023-05-13-surface-code/ 

[3]Google Quantum AI, Nature 614, 676–681 (2023).

[4]Google Quantum AI and Collaborators, Nature 638, 920–926 (2025).

日時：2025年10月1日15:00-

教室：31号館3階シミュレーション実験室

発表者1：高山 英梨（量子多体）

題目：4準位Förster共鳴を有するRydberg原子系における量子相の解析に向けて

概要：Rydberg原子系は、強い長距離相互作用を持ち、また原子の内部状態を個別に制御・観測できることから、アナログ量子シミュレータとして利用されている[1]。これまでに、スピン1/2の量子Ising模型[2]やXY模型[3]といった局所準位数が2である系の量子シミュレーションが実現されており、近年では局所準位数が3である系の量子シミュレーションが可能になった[4,5]。2025年には、二原子系において、局所的な4準位が関与するFörster共鳴によるラビ振動が観測された[6]。本研究は、今後多原子系への拡張が期待されるこの4準位系を対象とし、その量子相の解析を通して、新奇な量子多体現象の発現の可能性を探ることを目的としている。本発表では、まず二原子の一方に作用するlight shiftを加えることによって4準位系においてFörster共鳴が生じたという先行研究[6]を紹介する。次に、この系のハミルトニアンをスピン3/2の演算子を用いて構築した結果を報告する。

[1] A. Browaeys and T. Lahaye, Nat. Phys. 16, 132–142 (2020) 
[2] H. Bernien et al., Nature 551, 579-584 (2017)
[3] S. de Léséleuc et al., Science 365, 775-780 (2019)
[4] Y. Chew et al., Nat. Photon. 16, 724-729 (2022)
[5] M. Qiao et al., Nature 644, 889-895 (2025)
[6] G. Emperauger et al., Phys. Rev. A 111, 062806 (2025)

発表者2：寺前 遙哉（量子多体）

題目：Jaynes-Cummings-Hubbard模型で記述されるイオントラップ系におけるジョセフソン効果 

概要：Josephson効果とは、もともと超伝導接合系で発見された現象で、２つの超伝導体もしくは超流動体がトンネル効果で弱く結合すると、位相差に応じて電流もしくは粒子流が流れる現象である。希薄なBose気体からなる超流動体が薄いポテンシャル障壁で隔てられた系をボソニック・Josephson接合(BJJ)といい、BJJは２サイトのBose-Hubbard模型（BH模型）で定性的に記述される[1]。BJJの時に起こる現象として、Josephsonプラズマ振動（JPO）とセルフトラッピングなどが存在し、JPOとセルフトラッピングは、実際に冷却原子系の実験で観測されている[2]。

　今回の研究では２つのイオンをトラップした系に注目する。イオンの内部状態の内、特定の２つの状態をイオンの振動自由度（フォノン）とレッドサイドバンド結合することでJaynes-Cummings-Hubbard模型(JCH模型)で記述される系を実現できる[3]。JCH模型とBH模型はどちらも、２サイト上のBose粒子を記述する模型である。そのような類似点から、BJJで現れる現象がJCH模型で表されるイオントラップ系でも起こると予測される。JCH模型においてフォノン間相互作用を引き起こすレッドサイドバンド項とBH模型のオンサイト相互作用項は全く異なる形をしていることから、レッドサイドバンド項によってJosephson効果に関係する新しいダイナミクスが見つかることを期待している。そこで本研究では、２サイトのJCH模型におけるJosephsonダイナミクスを、平均場近似を用いて解析する。サイドバンド結合がフォノンのホッピングに比べて十分小さいという条件のもとで、摂動論を用いて、基底状態解を解析的に求める。得られた基底状態解からＪＰＯの振動数を与え、サイドバンド結合由来の相互作用エネルギーは４次の摂動という形で現れることを示す。

[1]S. Raghavan et al., Phys. Rev. A 59, 620 (1999)
[2]Michael Albiez et al., Phys. Rev.Lett.95,010402 (2005)
[3]占部伸二, 「個別量子系の物理-イオントラップと量子情報処理-」, 朝倉書店, 2017年

日時：2025年10月8日15:00-

教室：31号館3階シミュレーション実験室

発表者：Mathias Mikkelsen（QunaSys）

題目：An introduction to QSCI with a focus on time-evolved input states

概要：Quantum-selected configuration interaction (QSCI) utilizes an input state on a quantum device to select important bases (electron configurations in quantum chemistry) which define a subspace where we diagonalize a target Hamiltonian, i.e., perform selected configuration interaction, on classical computers. In this talk I will introduce the general method and briefly introduce various proposals for how to generate a relevant quantum state for QSCI sampling and how to determine if it is a “good” state.

The main focus will be on utilizing time evolution with the target Hamiltonian to prepare the input state, investigating the optimiality of the basis as a function of evolution time, Trotter approximation error etc. The scaling of resource requirements is investigated as a function of system size for a hydrogen chain. We also compare the result of a simple Hartree Fock initial state and an initial state given by an approximate UCCSD ansatz with classically determined parameters, showing that the latter can yield more compact basis sets than the former. Overall the time-evolved states investigated achieve basis sizes that are almost as compact as sampling the true ground state, without requiring knowledge of the ground state or iterative circuit optimization.

日時：2025年10月15日15:00-

教室：31号館3階シミュレーション実験室

発表者：段下 一平（量子多体）

題目：Correlation-spreading dynamics in optical-lattice systems loaded with ultracold atoms

概要：Ultracold atoms in optical lattices serve as a unique platform for studying quantum many-body dynamics far from equilibrium. In this work, we investigate dynamics of spatial correlation functions after a quantum quench starting from Mott insulating states in both Bose and Fermi gases in optical lattices, which are well described by Hubbard-type models, with a focus on how the correlations propagate. In the case of the Bose gases, using the projected entangled pair states (PEPS) algorithm, we compute time evolution of the single-particle and density-density correlation functions in two dimensions after a quantum quench starting from a Mott insulating state with unit filling [1]. Comparing our numerical results of the single-particle correlation function with the outputs from the experiments of Ref. [2], we show that PEPS can quantitatively capture the correlation-spreading dynamics of this system. We calculate the propagation velocities of the correlations for a wide range of the interaction parameter. In the case of the Fermi gases, we analyze dynamics starting from 1/N filled Mott insulators in the one-dimensional SU(N) Fermi-Hubbard model. From approximate analytical insights, we show that the velocity of correlation propagation increases with N due to the entanglement of doublon-excitation states and approaches 6J in the large-N limit, where J is the hopping energy. Using numerical simulations with matrix-product states, we calculate the time-evolution of the density-density correlation function in order to qualitatively confirm the analytical predictions.

[1] R. Kaneko and I. Danshita, Commun. Phys. 5. 65 (2022).

[2] Y. Takasu et al., Sci. Adv. 6, eaba9255 (2020).

[3] M. Mikkelsen and I. Danshita, in preparation.

日時：2025年10月29日15:00-

教室：31号館3階シミュレーション実験室

発表者：Sidharth Rammohan（量子多体）

題目：TBA

概要：TBA

日時：2025年11月12日15:00-

教室：31号館3階シミュレーション実験室

発表者：Jose Carlos Pelayo（量子多体）

題目：TBA

概要：TBA

日時：2025年11月19日15:00-

教室：31号館3階シミュレーション実験室

発表者：古内 理人（量子多体）

題目：TBA

概要：TBA

日時：2025年12月3日15:00-

教室：31号館3階シミュレーション実験室

発表者：植田 健太（量子多体）

題目：TBA

概要：TBA

日時：2025年12月10日15:00-

教室：31号館3階シミュレーション実験室

発表者：田中 愛梨（物性理論）

題目：TBA

概要：TBA

日時：2025年12月10日15:00-

教室：31号館3階シミュレーション実験室

発表者：川崎 大生（量子多体）

題目：TBA

概要：TBA

過去のセミナー

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/r^b . 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：Anomalous Tunneling of Spin-Wave Excitations in a Long-Range Interacting XY Model

概要： Various long-range interacting spin systems have been realized in Rydberg atom arrays and trapped ions, enabling experimental investigations of collective excitations such as spin waves and topological excitations [1–4]. In this work, we theoretically study the tunneling properties of low-energy spin-wave excitations through a potential barrier in a long-range interacting ferromagnetic XY model. Previous theoretical studies [5] have shown that, in short-range interacting ferromagnetic Heisenberg models, spin-wave excitations exhibit anomalous tunneling: the transmission probability increases as the excitation energy decreases, reaching perfect transmission in the zero-energy limit [6]. These excitations are classified as Nambu–Goldstone modes resulting from the spontaneous breaking of continuous symmetry. Reference [5] further suggests that Nambu–Goldstone modes can exhibit anomalous tunneling under certain conditions.

In this work, we theoretically explore the impact of long-range spin-spin interactions, whose strength depends algebraically on the distance r as ~ 1/r^β, on the anomalous tunneling of low-energy excitations. Using a mean-field approach [7], we numerically compute the transmission probability as a function of excitation energy and confirm that anomalous tunneling indeed occurs in the presence of long-range interactions. In addition, we identify distinctive signatures of long-range interactions: the reflection probability and the half-width of the zero-energy transmission peak exhibit power-law behavior, with exponents characteristic of the long-range interactions. Assuming that the decay exponent β can be tuned as demonstrated in trapped-ion system [4], we also investigate how the reflection probability and the half-width depend onβ.

[1] C. Chen et al. Nature, 616, 691 (2023).

[2] G. Bornet, Ph.D. thesis, Université Paris-Saclay (2024).

[3] P. Richerme et al., Nature 511, 198 (2014).

[4] N. Kotibhaskar et al., Phys. Rev. Research 6, 033038 (2024).

[5] Y. Kato, S. Watabe, Y. Ohashi, Journal of Physics: Conference Series 400, 032036 (2012).

[6] Yu. Kagan et al., Phys. Rev. Lett. 90, 130402 (2003).

[7] I. Danshita and D. Yamamoto, Phys. Rev. A 82, 013645 (2010).

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：Observation of Two-Polariton Bound States and Analysis of Detuning Dependence

概要：Trapped-ion enables the construction of well-isolated quantum systems consisting of several to tens of ions, minimizing the influence of external disturbances. With advances in laser cooling, ions can now be stably confined for long durations, making them ideal candidates for quantum bits.

In our laboratory, we implement quantum simulations of the Jaynes-Cummings-Hubbard (JCH) model [1, 2], which shares key features with the Bose–Hubbard model [3]. Focusing on the dynamics of polaritons—quasiparticles formed through coupling between internal states and phonons—we experimentally observe a two-polariton bound state (TPBS) and its hopping behavior using two ions.

Such repulsively bound localization has previously been observed only in ultracold atomic systems [4], and this is the first experimental realization in a trapped-ion platform. This phenomenon, arising from the discreteness of quantum energy levels, demonstrates the capability of trapped-ion quantum simulators to explore microscopic quantum effects.

We will also discuss future directions toward establishing an evaluation framework for quantum phase transitions in a JCH simulator based on two measurement techniques [5, 6].

[1] A. D. Greentree et al., Nat. Phys. 2, 856 (2006).

[2] D. G. Angelakis et al., Phys. Rev. A 76, 031805 (2007).

[3] J. Hubbard, Proc. R. Soc. London, Ser. A 276, 238 (1963).

[4] K. Winkler et al., Nature 441, 853 (2006).

[5] R. Ohira et al., Phys. Rev. A 100, 060301(R) (2019).

[6] S. Muralidharan et al., Phys. Rev. A 104, 062410 (2021).

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：Re-examination of electron correlations in the mechanism of superconductivity in the cuprates

概要：Since the discovery of high-temperature superconductivity　in the cuprates by Müller and Bednorz, various theories have been proposed, but the mechanism of superconductivity still cannot be fully explained. Motivated by a marked effect of BEC on the atomic correlation in He, a paper aims to elucidate the mechanism of superconductivity in the cuprates by re-examining the role of electron correlation[1].
I introduce the background and the detail of the experiment.

[1] T. Egami,Physica C 613,1354345(2023)

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：川崎 大生（量子多体）

題目：Collective Excitations in a Spin-1 XY Model with Long-Range Interactions

概要：Rydberg atom arrays have been successfully applied to quantum simulation of quantum many-body physics thanks to strong long-range interatomic interactions, and individual control and detection of internal atomic states [1]. Recent experiments have realized Förster-resonant coupling among three distinct Rydberg states [2,3], allowing for quantum simulation of models with effective spin-1 degrees of freedom [4]. Motivated by this development, we theoretically investigate the collective excitations in a spin-1 XY model with a quadratic Zeeman term and long-range spin-exchange interactions. Specifically, using the Schwinger-boson representation [5,6], we analyze the Nambu-Goldstone (NG) and Higgs modes in the XY ferromagnetic ordered phase near the quantum phase transition to the disordered phase. We show that the excitation energy of the gapless NG mode is proportional to the square root of the momentum, as is also the case in XY models with no quadratic Zeeman term [7,8,9], while the gapped Higgs mode exhibits a linear dispersion.

References:

[1] A. Browaeys and T. Lahaye, Nat. Phys. 16, 132–142 (2020)  
[2] Y. Chew et al., Nat. Photonics 16, 724–729 (2022)  
[3] M. Qiao et al., arXiv:2501.08233.
[4] T. Yoshida et al., arXiv:2409.08497.
[5] E. Altman and A. Auerbach, Phys. Rev. Lett. 89, 250404 (2002).
[6] K. Nagao and I. Danshita, Prog. Theor. Exp. Phys. 2016, 063I01 (2016)  
[7] D. Peter et al., Phys. Rev. Lett. 109, 025303 (2012)  
[8] O. K. Diessel et al., Phys. Rev. Research 5, 033038 (2023)
[9] C. Chen et al., Science 389, 483 (2025).