今期の予定
日時：2026年4月8日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：Sidharth Rammohan（量子多体）
題目：Cluster mean-field analysis on quantum dynamics in a planar array
of hardcore Bose-Hubbard chains with interchain interactions
概要：Dipolar bosons in optical lattices offer a versatile platform for quantum simulation, as their long-range and anisotropic interactions give rise to strongly correlated many-body phenomena. These properties have attracted significant experimental and theoretical interest in the study of emergent quantum phases [1,2]. In one-dimensional (1D) settings, such systems exhibit a collective behavior that can be effectively described by Luttinger liquid (LL) theory. Extending this framework to arrays of weakly coupled one-dimensional chains naturally raises the question of how interchain interactions lead to new quantum phases. In this context, the sliding Luttinger liquid (SLL) phase has been proposed as a stable state of weakly coupled 1D chains, where each chain retains its Luttinger-liquid character while interchain interactions enable phase sliding without establishing long-range order [3]. Realizing the SLL phase experimentally is challenging, as it requires strong interchain interactions while keeping the fluctuations minimal. Dipolar ultracold atomic systems, with their tunable long-range interactions, offer a promising platform for observing such a phase, and have consequently motivated theoretical studies exploring planar arrays of dipolar bosons as a route to realizing it [4]. However, existing theoretical studies have focused on equilibrium properties, and real-time dynamics of these systems remain relatively unexplored.
日時：2026年4月15日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：Jose Carlos Pelayo（量子多体）
題目：Multipolar structure in a Rydberg chain with a Four-level Förster resonance
概要：We investigate a chain of four-level Rydberg atoms exhibiting both two-level and four-level Förster resonances, the latter enabled by a local light field [1]. We analyze the system using a mean-field approach based on an effective spin-1/2 representation near the degenerate levels, and compare it with a full spin-3/2 mean-field treatment. Within the effective spin-1/2 description, we identify XY ferromagnetic, XY antiferromagnetic phases [2] and Ising Néel. Mapping these states back to the full spin-3/2 Hilbert space reveals an associated multipolar structure, that is pronounced near the four-level Förster resonance. These results are compared against matrix product state (MPS) calculations.
[1] Emperauger, Gabriel, et al. "Benchmarking direct and indirect dipolar spin-exchange interactions between two Rydberg atoms." Physical Review A 111 no. 6 (2025)
[2] Chen, Cheng, et al. "Continuous symmetry breaking in a two-dimensional Rydberg array." Nature 616 no. 7958 (2023)
日時：2026年4月22日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：古内 理人（量子多体）
題目：Domain formation dynamics in Rydberg-atom arrays described by a mixed-field Ising model on a square lattice
概要：Rydberg-atom systems have recently attracted considerable attention as highly controllable platforms for quantum simulation and quantum computation. Because both the spatial arrangement of atoms and the interactions induced by Rydberg excitation can be controlled, these systems provide a suitable platform for investigating nonequilibrium dynamics. In particular, a Rydberg-atom system can be described by mapping the ground and excited states of each atom onto pseudospin states can be described by a S=1/2 mixed-field Ising model [1]. Recent experiments [2] using such a Rydberg-atom system have addressed quantum dynamics after a quantum quench of the transverse and longitudinal fields starting from a fully polarized (all spin-down) state. It has been found that when a resonance condition called facilitation condition is satisfied, the cluster size of up-spin domains significantly grows in time [2].
In this study, aiming to understand mechanisms of the observed domain-formation dynamics, we theoretically investigate the quench dynamics in a relatively small system, where we can diagonalize the full Hamiltonian of the system and calculate the time evolution of an initial state using the resulting eigenvalues and eigenvectors. We discuss characteristic features of the time evolution by comparing them with the predictions of a pseudo-analytical perturbation theory that treats the transverse field as a perturbation.
[1]A. Browaeys and T. Lahaye, Nat. Phys. 16, 132 (2020).
[2] Philip Osterholz et al. arXiv preprint arXiv:2512.04656v2 (2026).
日時：2026年5月7日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：奥 奏太（物性理論）
題目：Numerical Analysis of Two-Dark-Soliton Collisions in a Driven-Dissipative Exciton-Polariton Condensate and Improvement of an Effective Equation of Motion
概要：Exciton-polariton condensates are nonequilibrium condensates formed through strong coupling between excitons and cavity photons in semiconductor microcavities [1]. Owing to their small effective mass, macroscopic coherence, and nonlinear interactions, they provide a useful platform for studying nonequilibrium quantum-fluid dynamics. Dark solitons are an important class of nonlinear excitations in such condensates. In contrast to conservative condensates, however, polariton condensates have finite lifetimes and are maintained by a balance between external pumping and dissipation. This driven-dissipative nature modifies the lifetime and dynamics of dark solitons [2]. In this study, we numerically analyze collisions between two dark solitons in a one-dimensional driven-dissipative exciton-polariton condensate.
We solve the coupled partial differential equation (PDE) system consisting of a driven-dissipative Gross-Pitaevskii equation and a reservoir-density equation under periodic boundary conditions. From the numerical density profiles, we track local density minima and extract the inter-soliton separation η(t). We then compare η_PDE(t), obtained from the PDE simulations, with η_ODE(t), obtained from an effective equation of motion formulated as an ordinary differential equation (ODE) for the relative coordinate [3]. By sweeping the reservoir decay rate γ_R and the stimulated scattering coefficient R̄, we evaluate the root-mean-square error (RMSE) during the post-collision repulsive dynamics and visualize the parameter dependence of the agreement between the PDE and ODE results using heat maps. We also show that an improved treatment of the reservoir contribution, incorporating local density dependence, systematically reduces the RMSE in parameter regions where the inter-soliton separation can be reliably extracted.
参考文献:[1] S. Ghosh, R. Su, J. Zhao, A. Fieramosca, J. Wu, T. Li, Q. Zhang, F. Li, Z. Chen, T. C. H. Liew, D. Sanvitto, and Q. Xiong, Photonics Insights 1, R04 (2022).
[2] L. A. Smirnov, D. A. Smirnova, E. A. Ostrovskaya, and Y. S. Kivshar, Phys. Rev. B 89, 235310 (2014).
[3] Y. Zhang, C. Jia, and Z. Liang, arXiv preprint arXiv:2112.03559v1 (2021).
日時：2026年5月13日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：Edwin Chaparro (JILA, University of Colorado)
題目：Bidirectional Teleportation Using Scrambling Dynamics: A Practical Protocol
概要：We show that quantum information scrambling can enable a generic SWAP gate between collective degrees of freedom in systems without universal local control. Our protocol combines the Hayden–Preskill recovery scheme, associated with the black hole information paradox, with quantum teleportation and runs them in parallel and in opposite directions, enabling bidirectional exchange of quantum states through global interactions alone. This approach cleanly distinguishes the roles of information spreading, entanglement, and chaos for enabling both coherent state transfer and recovery. We propose an experimental realization using the Dicke model, which can be realized in cavity-QED and trapped-ion platforms, highlighting the utility of holography in designing practical quantum gates.
日時：2026年5月20日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：川崎 大生（量子多体）
題目：Higgs mode in a two-dimensional dipolar XY model with a staggered field
概要：Long-range interacting spin systems have attracted considerable attention owing to recent experimental advances in quantum platforms such as Rydberg atoms in optical tweezer arrays and trapped ions [1–5]. These developments have enabled the experimental exploration of quantum many-body dynamics, including collective excitations associated with symmetry breaking. Motivated by recent progress in quantum simulation using three Rydberg states [6,7], we have recently investigated Higgs and Nambu-Goldstone modes of a XY-ferromagnetic phase in a spin-1 XY model with long-range interactions and showed that the long-range nature of the interactions can significantly suppress the damping of the Higgs mode [8]. This result suggests a possible route toward realizing long-lived Higgs modes in Rydberg-atom arrays. However, since the spin-1 Rydberg-array system considered in Ref. [8] has not yet been realized experimentally, it will be useful to explore the Higgs-mode physics in other long-range interacting spin systems that have been already realized.
In this work, we theoretically investigate collective excitations in a spin-1/2 XY model with dipole-dipole interactions and a staggered magnetic field, which describes a certain system realized experimentally with Rydberg-atom arrays [2]. When the strength of the staggered field is varied, this model exhibits a quantum phase transition between the XY ferromagnetic and polarized phases. Since the universality class of the transition of this model is the same as that of the spin-1 model [9], it is highly expected that a Higgs mode emerges in the XY ferromagnetic phase near the transition. Using the Schwinger-boson representation [8,10,11], we analyze the excitation spectra of the collective modes in order to show that this is indeed the case. We also find that the Higgs mode exhibits a linear dispersion with a finite energy gap, qualitatively consistent with the behavior found in the spin-1 XY model with dipole-dipole interactions and a quadratic Zeeman term [8].
References:
[1] A. Browaeys and T. Lahaye, Nat. Phys. 16, 132 (2020).
[2] C. Chen et al., Nature 616, 691 (2023).
[3] C. Monroe et al., Rev. Mod. Phys. 93, 025001 (2021).
[4] P. Richerme et al., Nature 511, 198 (2014).
[5] N. Kotibhasker et al., Phys. Rev. Research 6, 033038 (2024).
[6] Y. Chew et al., Nat. Photon. 16, 724 (2022).
[7] M. Qiao et al., Nature 644, 691 (2025).
[8] D. Kawasaki and I. Danshita, arXiv:2512.24557 (2025), to be published in Phys. Rev. A.
[9] B. Sbierski et al., Phys. Rev. B 109, 144411 (2024).
[10] E. Altman and A. Auerbach, Phys. Rev. Lett. 89, 250404 (2002).
[11] K. Nagao and I. Danshita, Prog. Theor. Exp. Phys. 2016, 063I01 (2016).
日時：2026年6月3日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：庄野 滉平（場の量子論・素粒子論研究室）
題目：Introduction to Hawking Radiation via Quantum Field Theory
概要：In general relativity, a black hole is a region of spacetime from which nothing, not even light, can escape. In classical theory, black holes are regarded as objects that only absorb matter and radiation. However, by incorporating the effects of quantum field theory, Stephen Hawking theoretically demonstrated that black holes can emit thermal radiation, now known as Hawking radiation. This presentation aims to provide an introduction to this phenomenon. We first review the canonical quantization of a free scalar field in flat spacetime, where the field is interpreted as an infinite collection of harmonic oscillators. We then discuss the quantization of the Dirac field in order to clarify the differences between bosonic and fermionic fields.Building on this foundation, we extend the discussion to the quantization of scalar fields in curved spacetime. We show that, in a non-stationary spacetime background, a Bogoliubov transformation reveals a mismatch between the mode functions defining the past and future vacua. This mismatch leads to particle creation: even if the initial state is empty in the past, particles can be observed in the future stationary region. Through this mechanism, we explain the basic idea underlying Hawking radiation.
日時：2026年6月10日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：Mengxuan Xia（場の量子論・素粒子論研究室）
題目：An Introduction to PT-Symmetric Quantum Mechanics
概要：In conventional quantum mechanics, closed systems are governed by strict Hermitian axioms that ensure probability conservation. However, physically realistic systems continuously interact with their environment. This seminar explores fascinating PT-Symmetric Quantum Mechanics, moving beyond the rigid Hermitian boundaries of conventional physics. We demonstrate how coupling two time-reversed open subsystems mathematically forces a zero net probability flux, creating an intermediate regime between closed and open systems. By investigating both quantum matrix models and classical coupled oscillators, we reveal the unique topological signature of PT phase transitions: the merging of real energy levels. We prove that PT symmetry provides a profoundly broader mathematical condition than Hermiticity, expanding our search space for finding new, stable equilibrium systems.
日時：2026年6月17日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：吉田 啓（物性理論）
題目：Towards Understanding the Berezinskii–Kosterlitz–Thouless Transition in Two-Dimensional Bose Systems with Dipole–Dipole Interactions
概要：In general, it is well known that two-dimensional systems with continuous degrees of freedom do not exhibit long-range order at finite temperatures, and therefore do not undergo conventional phase transitions. However, in systems such as the two-dimensional XY model and helium thin films, a special type of phase transition known as the Berezinskii–Kosterlitz–Thouless (BKT) transition can occur, despite the absence of long-range order.
The BKT transition is a phase transition in which the presence of interactions within the system plays a crucial role. However, most previous studies of the BKT transition in cold atomic gases have considered only short-range and isotropic interactions, and the effects of long-range and anisotropic interactions on this phase transition have not yet been fully understood. Therefore, it is of great interest to investigate how the BKT transition behaves in two-dimensional systems with dipole–dipole interactions, which are characterized by their long-range and anisotropic nature.
In this presentation, after reviewing the fundamental theory of the BKT transition in the two-dimensional XY model and in helium thin films, I will introduce experimental results from previous studies that have investigated the BKT transition in two-dimensional dipolar Bose gases.
日時：2026年6月24日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：小林 麟太郎（量子多体）
題目：Towards a theoretical analysis of correlation propagation after a quantum quench in ultracold atomic gases in optical lattices
概要：Systems of ultracold atomic gases serve as a quantum many-body system isolated near-perfectly from environment and allow for precise control and detection of atomic ensembles [1]. Using these advantages, nonequilibrium quantum dynamics after a quantum quench, namely a sudden change of Hamiltonian parameters, has been studied extensively. In particular, the question of how quantum correlations spread in space after a quantum quench has been regarded as a fundamental question related to quantum-information propagation and thermalization in isolated quantum systems.
As experimental background, I first introduce the experiment by Cheneau et al. [2]. In one-dimensional (1D) Bose gases in optical lattices, which is quantitatively described by the 1D Bose-Hubbard model, they measured the time evolution of parity correlations with use of quantum-gas microscopy after a quench from the Mott-insulating regime, and observed a correlation signal spreading ballistically over time.
I then review the theoretical work by Barmettler et al. [3], which analyzes this correlation propagation using both analytical and numerical methods. In this talk, I focus on the strong-coupling regime, where the ground-state is the Mott insulator. In this regime, density fluctuations are suppressed, so the local Hilbert space can be truncated to three states: the average occupation, one extra particle, and one missing particle. By introducing auxiliary particles and using the Jordan-Wigner transformation together with the unconstrained-fermion approximation, the 1D Bose-Hubbard model is reduced to a quadratic effective Hamiltonian. On the basis of the quasiparticle picture obtained from this effective model, the correlation propagation after the quench can be interpreted as being carried by doublon-holon-like quasiparticle pairs. The propagation front is characterized by their relative velocity, and the negative dip in the density correlation is due to the contribution of these quasiparticle pairs.
[1] F.Schäfer et al., Nat. Rev. Phys. 2, 411 (2020).
[2] M.Cheneau et al., Nature 481, 484 (2012).
[3] P.Barmettler et al., Phys. Rev. A 85, 053625 (2012).
日時：2026年7月1日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：田中 愛梨（物性理論）
題目：Dynamics of superfluid wake behind plate-like obstacles
概要：This study investigates the superfluid wake dynamics behind a plate-like obstacle in Bose–Einstein condensate using numerical calculations based on the Gross–Pitaevskii equations. In classical fluids, the wake structure behind an obstacle is organized by the Reynolds number, transitioning from steady vortices at low Reynolds numbers to Karman vortex streets, irregular flow, and turbulence at high Reynolds number. On the other hand, since superfluids do not have viscosity, the classical Reynolds number cannot be directly applied. However, when the obstacle velocity exceeds the critical velocity, quantum vortices are generated, and unsteady wakes accompanied by dissipation appear [3]. Therefore, it is important to clarify whether there are dimensionless quantities or universal behaviors that govern the wake state when the size and velocity of the obstacle are changed, even in superfluids [1] [2].
This study analyzed the condensate density, phase distribution, and quantum vortex trajectories of a plate-like obstacle, varying its size and the drag force acting on the obstacle was calculated from the law of conservation of momentum, and the drag coefficient was calculated from the time-averaged value after reaching a steady-state oscillation. Finally, power spectral analysis was performed on the time series of the drag force, and the Strohll number was determined from the major vortex emission frequencies.
Calculations revealed that the wake behind the plate-shaped obstacle changes from vortex-free, vortex-present, bubble-like structure, to an irregular state, depending on the velocity and obstacle size. In particular, even at the same velocity, the wake region expands as the obstacle size increases, and the quantum vortex generation and emission process becomes more complex. Furthermore, in the high superfluid Reynolds number region, the drag coefficient converges to a nearly constant value regardless of obstacle size, and the Strohll number exhibits similar behavior to that obtained with cylindrical obstacles, showing a tendency to approach a constant value as the superfluid Reynolds number increases. These results suggest that the vortex emission frequency in plate-shaped obstacles may also be organized by the superfluid Reynolds number, similar to the case of cylinders.
References
[1] K. Sasaki et al., “Bénard–von Kármán Vortex Street in a Bose-Einstein Condensate,” Physical Review Letters 104, 150404 (2010).
[2] M. T. Reeves et al., “Identifying a Superfluid Reynolds Number via Dynamical Similarity,” Physical Review Letters 114, 155302 (2015).
[3] H. Kokubo and K. Kasamatsu, “Critical Velocity for Quantized Vortex Formation in a Superfluid with a Plate-Shaped Obstacle,” Physical Review A 111, 043314 (2025).
日時：2026年7月8日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：百合 巧（大阪大学）
題目：Potential control and vibrational excitations in trapped-ion systems for many-body quantum simulation
概要：Trapped-ion systems provide a controllable platform for quantum simulation based on internal and motional degrees of freedom. In this talk, I will present our recent efforts toward many-body quantum simulation with trapped ions, focusing on potential control and vibrational excitations.
First, I will discuss potential control in many-ion crystals. For quantum simulation using motional excitations, it is important to prepare trap conditions in which the desired radial excitation can be selectively addressed. To this end, we adjust voltages applied to the control electrodes to modify the trap potential and search for conditions where the relevant radial sideband spectra are sufficiently separated. In many-ion crystals, however, an inappropriate process of changing the trap potential can cause melting of the ion string, structural rearrangement, or loss of ions. We therefore investigate potential control for reaching the target trap conditions while maintaining the ion crystal.
Second, I will discuss polaritonic excitations formed by coupling radial phonons to internal electronic excitations using sideband optical pulses. In our previous report [1], we observed indications of detuning-dependent dynamics in two-polariton bound states. Here, I will present measurements over a wider range of detuning and discuss how the bound-state dynamics change with detuning.
These studies provide complementary steps toward many-body quantum simulation with trapped ions, including Jaynes-Cummings-Hubbard-type quantum simulators.
References
[1] T. Yuri, “Observation of Two-Polariton Bound States and Analysis of Detuning Dependence,” Kindai QPT Seminar, July 9, 2025.
日時：2026年7月15日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：吉本 康晟（量子多体）
題目：TBA
概要：TBA
日時：2026年7月22日15:00-
教室：31号館4階数物会議室+Zoom配信
発表者：渡部 元輝（分子科学研究所）
題目：Implementation of a Two-Qubit Rydberg Gate on a Neutral-Atom Quantum Computer
概要：Neutral-atom platforms have emerged as a highly scalable architecture for a universal quantum computer. Implementing high-fidelity two-qubit entangling gates is one of the crucial requirements to execute quantum computations. The neutral-atom quantum computer implements the two-qubit gate by using a highly excited Rydberg state, its coherent operation, and long-range interaction [1, 2].
In this study, we report on the comprehensive development, characterization of the laser system and successful experimental demonstration of a two-qubit Rydberg gate on a neutral-atom quantum computer. First, we evaluate several laser sources and select those satisfying low phase noise, which is one of the major error sources in coherent manipulation. Next, we construct a frequency stabilization system based on an ultra-low-expansion (ULE) optical cavity to achieve the narrow linewidth required for coupling to the Rydberg state. To optimize the two-qubit gate pulses, we develop the tandem acousto-optic modulator (AOM) system that enables high power operation and fast pulse shaping. Furthermore, laser beams are locally addressed to the target qubits using an acousto-optic deflector (AOD). With this laser system, we successfully realize coherent Rydberg coupling. Finally, by exploiting the long-range Rydberg interaction, we experimentally observe the Rydberg blockade effect between atom pairs and achieve Bell-state generation using two-qubit entangling gate. These results represent an important step toward the realization of high-fidelity, fault-tolerant universal quantum computers.
[1] D. Jaksch et al. “Fast Quantum Gates for Neutral Atoms”. In: Phys. Rev. Lett. 85 (10 Sept. 2000), pp. 2208–2211.
[2] Evered, S. J. et al. High-fidelity parallel entangling gates on a neutral-atom quantum computer. Nature 622, 268–272 (2023).
日時：2026年7月29日13:15-
教室：31号館4階数物会議室+Zoom配信
発表者1：田邊 魁馬（東京理科大学）
題目：Evaluation of Energy Barriers across the BCS–BEC Crossover in a Ring-Trapped Superfluid Fermi Gas
概要：In superfluids, long-lived dissipationless flows, known as persistent currents, can exist. However, at finite temperatures, these flows can decay via thermally activated phase slips. The decay rate is described by an Arrhenius relation and is governed by the energy barrier separating a metastable superflow state from an unstable steady state. Evaluating this barrier requires identifying unstable solutions, which is inherently challenging using conventional numerical methods. For Bose–Einstein condensates (BECs), unstable solutions of the Gross–Pitaevskii equation have been obtained using the pseudo-arclength continuation method [1], as demonstrated in Refs. [2–4]. However, in superfluid Fermi gases, only metastable states have been explored [5], because applying the pseudo-arclength continuation method to the Bogoliubov–de Gennes (BdG) equations is not straightforward. Thus, a systematic evaluation of energy barriers involving unstable steady-state solutions in ring-trapped Fermi gases remains lacking.