近畿大学理工学部物理学コースの量子制御研究室、量子多体物理学研究室、物性理論研究室が合同で運営する量子物理学および量子技術に関するセミナーです。学期中は基本的に毎週水曜日10:45-12:15に開催しています。

今後の予定

2022年度



日時: 2022年12月7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: 鏡原 大地(量子多体)

題目: Entanglement entropy in a non-Hermitian linear optical system

概要: 

Sampling complexity refers to the difficulty of sampling from a distribution close to the desired (e.g., quantum system) probability distribution and is sometimes discussed in the context of quantum supremacy. Recently, by considering the sampling problem of identical bosons time-evolving by a quadratic Hamiltonian from a product state, a transition from the state in which sampling can be done easily to that where it is difficult has been proposed[1]. Furthermore, it has been pointed out that when time evolution is extended to be non-unitary by considering open systems, there is a transition in sampling complexity that has not been observed in unitary systems[2].

 

The aim of this work is to clarify the relation between the sampling complexity and other complexity measures, such as entanglement entropy. We extend the method to calculate the Rényi entanglement entropy in a unitary non-interacting boson system[3] to the non-Hermitian case. We apply our method to the non-Hermitian situation discussed in Ref. [2] and calculate the Rényi entanglement entropy. We will discuss how the scaling law of the entanglement entropy and sampling complexity are related.

 

[1] A. Deshpande, B. Fefferman, M. C. Tran, M. Foss-Feig, and A. V. Gorshkov, Physical Review Letters 121, 030501 (2018).
[2] K. Mochizuki and R. Hamazaki, arXiv.2207.12624.
[3] D. Kagamihara, R. Kaneko, S. Yamashika, K. Sugiyama, R. Yoshii, S. Tsuchiya, and I. Danshita, arXiv:2207.08353.


日時: 2022年12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: 久木田 真吾(量子制御)

題目: Lower bound on operation time of composite pulses robust against pulse length error

概要: Precise control of quantum systems is a cornerstone for realizing high-quality quantum technology such as quantum computing and quantum communication. The performance of control of systems often deteriorates due to systematic errors. In one-qubit control, the pulse length error (PLE) is one of typical systematic errors, which is caused by deviation of the strength of a control field. A composite pulse (CP) is a method for suppressing the effects of such systematic errors. A CP is a sequence of operations, where systematic errors in the operations cancel each other. Many CPs that are robust against the PLE have been found. However, their operation time tends to be longer than a simple one because CPs replace a simple operation with multiple ones. A longer operation time implies stronger decoherence, and thus a shorter CP is preferable from the viewpoint of noise immunity. In this talk, we discuss a lower bound on the operation time of CPs robust against the PLE. This bound is not saturable, but non-trivial.


日時: 2022年12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: 國見 昌哉(東京理科大学)

題目: Proposal for realization of the quantum-spin chain with Dzyaloshinskii-Moriya interaction using Rydberg atom quantum simulators

概要: Recently, the combination of optical tweezers and Rydberg atoms has enabled the realization of highly-controllable quantum simulators for quantum spin systems, and various quantum many-body phenomena have been studied. Typical quantum spin models such as the S=1/2 Ising, XY, and XXZ models have been experimentally realized by using Rydberg atoms with optical tweezers [1]. In some actual magnets, the Dzyaloshinskii-Moriya (DM) interaction exists, which originates from spin-orbit couplings. It is well known that the DM interaction causes various nontrivial phenomena. For example, skyrmion [2] and chiral soliton lattice [3] emerge due to the DM interactions. The DM interaction has not been realized in the Rydberg atom quantum simulators.

In this talk, we propose a method to experimentally realize a 1D quantum spin model with the DM interaction using the Rydberg atom systems.
Specifically, a Rydberg state (e.g., |nS>) is coupled to another Rydberg state (|n’S>) by a two-photon Raman transition. The Rabi coupling with different phases at each atom position can be realized by setting the direction of the lasers appropriately.  By unitary transformation of this system and transformation to the rotating frame, we can obtain a spin Hamiltonian with the DM interaction.

In addition to the experimental proposal, we will present the numerical results for the DH model [4], whose Hamiltonian has the DM interaction and transverse magnetic field only. We investigate the nonequilibrium dynamics of quantum solitons in the DH model and find that the quantum solitons decay after a long-time evolution. Finally, we also show that the DH model has quantum many-body scar states by explicitly constructing a restricted spectrum generating algebra [5].

[1] A. Browaeys and T. Lahaye, Nature Phys. 16, 132 (2020).
[2] N. Nagaosa and Y. Tokura, Nature Nanotechnology, 8, 899 (2013).
[3] J. Kishine and A. S. Ovchinnikov, Solid State Phys. 66, 1 (2015).
[4] S. Kodama, A. Tanaka, and Y. Kato, arXiv:2209.04227 (2022).
[5] S. Moudgalya, N. Regnault, and B. A. Bernevig, Phys. Rev. B 102, 085140 (2020).


日時: 2023年2月8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: Mikkelsen, Mathias(量子多体)

題目: TBA

概要: TBA


 

 

過去の講演

2022年度前期


日時: 2022年4月20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: 小久保 治哉(物性理論)

題目: Dynamics of the wake in the Gross-Pitaevskii model with a small nonlinear coefficient

概要: 

When an object moves at a constant speed inside a fluid, a wake appears behind the object depending on the size and speed of the object. In a cold atomic gas Bose-Einstein condensate (BEC), quantum vortex generation in the wake of an obstacle has been observed when the velocity of the obstacle potential exceeds the critical velocity [1-2].
The critical velocity is strongly dependent on the shape of the obstacle potential. The critical velocity is about $0.37$ times the speed of sound if the obstacle size is sufficiently larger than the healing length of the superfluid [3-5]. Furthermore, when the obstacle is delta-functional, the critical velocity $v_c$ is near the speed of sound. On the other hand, the critical velocity is zero in the limit that the nonlinear coefficient of the Gross-Pitaevskii equation describing the motion of the cooled atomic gas BEC is zero (linear Schrodinger limit). In this talk,we show that the critical velocity decay with decreasing the nonlinear coefficient and investigate the associated dynamics of the wake and the quantum vortex generation.

[1] G. W. Stagg, N. G. Parker, and C. F. Barenghi, J. Phys. B 47, 095304 (2014).

[2] K. Sasaki, N. Suzuki, and H. Saito, Phys. Rev. Lett. 104, 150404 (2010).

[3] S. Rica, Physica D 148, 221 (2001).

[4] C. T. Pham, C. Nore, and M.E. Brachet, Physica D 210, 203 (2005).

[5] W. J. Kwon, G. Moon, S. W. Seo, and Y. Shin, Phys. Rev. A 91, 053615 (2015). 


日時: 2022年4月27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: 鏡原 大地(量子多体)

題目: Finite temperature phase diagram of a three-dimensional spin-dependent Fermi Hubbard model

概要: Recent experimental developments in ultracold atomic physics enable us to simulate various interesting many-body systems. Combining state-dependent optical lattices [1,2], Feshbach resonances [3], and Rabi coupling between two internal states induced by microwaves or lasers [1,2], one can realize a spin-dependent Fermi Hubbard model (SDFHM) [4].

In this work, we investigate a finite temperature phase diagram of a three-dimensional SDFHM with an attractive interaction. In the ordinary Fermi Hubbard model, the so-called Bardeen-Cooper-Schrieffer (BCS)-Bose-Einstein Condensate (BEC) crossover phenomenon is expected. We first review the BCS-BEC crossover based on the Nozières-Schmitt-Rink (NSR) theory [5,6] in the standard Hubbard model. We extend the NSR approach to SDFHM and discuss how differences in hopping amplitudes and Rabi coupling affect the superfluid phase transition temperature.

[1] L. Krinner, M. Stewart, A. Pazmino, J. Kwon, and D. Schneble, Nature 559, 589 (2018).
[2] L. Riegger, Ph.D. Thesis (2019).
[3] C. Chin, R. Grimm, P. Julienne, and E. Tiesinga, Rev. Mod. Phys. 82 1225 (2010).
[4] W. V. Liu, F. Wilczek, and P. Zoller, Phys. Rev. A 70, 033603 (2004).
[5] P. Nozières and S. Schmitt-Rink, J. Low. Temp. Phys. 59, 195 (1985).
[6] H. Heiselberg, in “The BCS-BEC Crossover and the Unitary Fermi Gas”, edited by W. Zwerger (Springer, New York, 2011). 


日時:2022年5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: 久木田 真吾(量子制御)

題目: Short composite operation robust against two common systematic errors

概要: Unavoidable systematic errors hinder precise quantum control. Pulse length error (PLE) and off-resonance one (ORE) are typical systematic errors that are encountered during one-qubit control. A composite operation, one of open-loop error cancellation techniques, can help compensate for the effects of systematic errors during quantum operation. Several composite operations that are robust against either PLE or ORE have been identified. However, few attempts have been made to construct composite operations that are robust against both errors (bi-robust) simultaneously. We develop a new bi-robust composite operation for controlling one-qubit, which exhibits a short operation time [1].

[1] S. Kukita, H. Kiya, Y. Kondo, arXiv: 2112.12945 [quant-ph]


日時:2022年5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: 數田 裕紀(量子多体)

題目: Towards quantum simulation of non-ergodic behavior of the one-dimension Bose-Hubbard model with a trapping potential

概要: The recent development of ultracold gas experiments allows us to simulate the thermalization of isolated quantum systems. When the system satisfies the eigenstate thermalization hypothesis (ETH) [1], it thermalizes after long-time evolution. Integrable systems [2] and many-body localized systems (MBL) with random potentials [3] are known as the systems that do not satisfy the ETH. Recently, violation of the ETH has been observed in systems that neither are integrable nor have random potentials [4,5]. Typical examples include quantum many-body scar [4] and stark MBL [5]. In this presentation, I focus on another system that violates the ETH. I specifically investigate the time evolution of the initial state  in the one-dimensional ultracold atoms in the trap potential. I will first reproduce the results of a previous study [6] numerically. Then, I will present some preliminary results of our attempts for observing the non-ergodic behavior with the use of a remote quantum simulator developed by the Quantum Optics group at Kyoto University.

[1] J. M. Deutsch, Phys. Rev. A 43, 2046 (1991).

[2] T. Kinoshita , T. Wenger & D. S. Weiss, Nature 440 900 (2006).

[3] J. Choi, S.Hild, et al. Science 352, 1547(2016)

[4] C. J. Turner, A. A. Michailidis, et al, Nat. Phys. 14, 745 (2018).

[5] E. van Nieuwenburg, Y. Baum, et al, PNAS 116, 9269 (2019).

[6] M. Kunimi and I. Danshita, Phys. Rev. A 104,043322 (2021).


日時:2022年5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: Mikkelsen, Mathias(量子多体)

題目: Noise correlations and spin-structure factor for the SU(N) Hubbard model

概要: The realization of SU(N) Hubbard models experimentally using Alkaline-earth atoms [1] has lead to renewed theoretical interest in this model, see e.g. [2] for a review of the phase diagram. The spin-spin correlations are a natural probe for the SU(N) order in the system and measuring them experimentally is therefore important.  It is well-established that the noise correlations measured by time-of-flight experiments, which corresponds to the momentum fluctuations of the initial state, can probe the spin structure factor in the Mott-limit of SU(2)-Hubbard models [3].In this talk I explicitly establish the mathematical relationship between the momentum-momentum fluctuations and spin structure factor for Mott-insulating states in the SU(N)-Hubbard model at any integer filling. The relation is confirmed numerically by DMRG calculations for 1D Fermi-Hubbard models with N=2-6 and it is shown that the momentum fluctuations still reflect the SU(N) order for weaker interactions and incommensurate filling (i.e. in the metallic phases away from the Mott-limit).

[1] A.V. Gorshkov et al. Nature Physics 6, 289–295 (2010)
[2] S. Capponi, P. Lecheminant and K. Totsuka, Ann.Phys. 367, 50 (2016)
[3]  E. Altman, E. Demler, and M. D. Lukin, Phys. Rev. A 70, 013603 (2004) 


日時:2022年6月1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: 中村 優希(京都大学)

題目: Rydberg atoms described by the Ising model with sign-inverted next-nearest-neighbor interaction

概要: In recent years, taking advantages of their high controllability, Rydberg atoms confined in an optical tweezer array have been utilized as a quantum simulator of quantum spin models [1,2]. Controllable quantum simulators using up to 256 neutral atomic arrays have been realized so far, and new quantum phases and related phase transitions have been experimentally observed [2]. In the case of quantum simulators of Ising-type models, the interaction between two spins that has been realized thus far is of van der Waals type, in which the sign of the nearest neighbor interaction is the same as that of the next-nearest neighbor one.

In this talk, I propose a method to realize Ising model with sign-reversed next-nearest neighbor interactions by weakly coupling one Rydberg state to another Rydberg state. I also discuss surface criticality [3,4] as an example of interesting phenomena that can occur in this novel system. I derive Ginzburg-Landau (GL) equation describing the motion of antiferromagnetic order parameters near the first-order phase transition point. By comparing it with numerical calculations of microscopic models in the mean-field approximation, we verify the validity of the analytical calculation based on the derived GL equation for surface criticality.

[1] M. Endres et al., Science 354,6315 (2016).

[2] S. Ebadi et al., Nature 595, 227 (2021).

[3] R. Lipowsky, Phys. Rev. Lett. 49, 1575 (1982).

[4] I. Danshita et al., Phys. Rev. A 91,013630 (2015).


日時:2022年6月8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: 宮井 誠一郎(量子多体)

題目: Towards analysis of correlation propagation velocities in the SU(N) Hubbard model

概要: Quantum simulation means mimicking a quantum many-body system of interest by using another system with high controllability. Since its first proposal by Feynman in 1981 [1], quantum simulation has been realized in various experimental systems [2]. One of the most promising applications of quantum simulation is to analyze non-equilibrium dynamics of quantum many-body systems in the sense that it is in general difficult with classical computers [3]. For example, in a quantum simulator consisting of a Bose gas in an optical lattice, the time evolution of equal-time correlation functions has been measured and a mechanism of the correlation propagation based on a quasiparticle picture has been discussed [4]. These quasiparticles are composed of excess particles (doublon) and holes (holon), which can be described as slave-bosons [5].

The purpose of this study is to analyze how the propagation velocity of the correlation in the SU (N) Hubbard model behaves by using the slave-boson method. In this presentation, as a preliminary step for this purpose, we review the derivation of the maximum group velocity, which is serves as an estimation of the upper limit of the information propagation velocity in the one-dimensional Bose-Hubbard model, on the basis of the method using auxiliary bosons [5], and the auxiliary bosons method for the Fermi-Hubbard model [6].

 

[1] R. P. Feynman, Int. J. Theor. Phys. 21, 467 (1982).

[2] I. M. Georgescu et al., Rev. Mod. Phys. 86, 153 (2014).

[3] K. Nagao and I. Danshita, Suurikagaku 684, 36 (2020), in Japanese.

[4] M. Cheneau et al., Nature 481, 484 (2012).

[5] P. Barmettler et al., Phys. Rev. A 85, 053625 (2012).

[6] G. Kotliar and A. E. Ruckenstein, Phys. Rev.Lett. 57, 1362 (1986).


日時:2022年6月15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: 木屋 晴貴(量子制御)

題目: General off-resonance-error-robust symmetric composite pulses with three elementary operations

概要: The performance of quantum computing is highly dependent on the accuracy of quantum control in each process.Realistic quantum operations suffer from undesirable effects of systematic errors caused by the miscalibration of experimental apparatuses and such errors deteriorate the performance of quantum control.In one-qubit control, one mainly confronts two typical systematic errors: pulse length error (PLE) and off-resonance error (ORE).A composite pulse (CP) is a method used to compensate for such systematic errors.For one-qubit operations, several CPs that are robust against PLE have been found, such as SK1[1], BB1[2] and SCROFULOUS[3].On the other hand, ORE-robust CPs that implement arbitrary \(\theta\)-rotations have been less investigated. The CORPSE family is one of the simplest and most tractable ORE-robust CPs for implementing arbitrary \(\theta\)-ratations[4].

 In this talk, I explain a systematic construction of a wide class of ORE-robust CPs that implement arbitrary \(\theta\)-rotations.In this class of CPs, we have one continuous free parameter to choose a CP that implements a target operation. We evaluate the performance of the CPs in this class in terms of gate fidelity(and infidelity) and the time required for operation[5].

 
References:
[1] K. R. Brown, A. W. Harrow, and I. L. Chuang,Phys. Rev. A \textbf{70}, 052318 (2004); K. R. Brown,A. W. Harrow, and I. L. Chuang, Phys. Rev.A \textbf{72}, 039905(E) (2005).
[2] Wimperis, S. 1994 Broadband, narrowband, and passband composite pulses for use in advanced NMR experiments. J. Magn. Reson. A \textbf{109}, 221–231. (doi:10.1006/jmra.1994.1159)
[3] Cummins, H. K. Llewellyn, G. , Jones, J. A. 2003 Tackling systematic errors in quantum logic gates with composite rotations. Phys. Rev. A \textbf{67}, 042308. (doi:10.1103/PhysRevA.67.042308)
[4] H. Cummins and J. Jones, Use of composite rotations to correct systematic errors in nmr quantum computation, New Journal of Physics \textbf{2}, 6 (2000).
[5] S. Kukita, H. Kiya, Y.Kondo, arXiv:2203.05754[quant-ph]

日時:2022年6月24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: 市川 翼(大阪大学 量子情報・量子生命研究センター)

題目: Bayenianism, Conditional Probability and Laplace Law of Succession in Quantum Mechanics

概要: The axioms of quantum mechanics (QM) assume the existence of a measurer.
Although this is not a problem in practical terms, it has often been pointed out
by Heisenberg and others that this is a fundamental feature that distincts
QM from classical mechanics.
 QBism (Quantum Bayesianism) is a recent attempt of reconstruction of QM
with emphasis on this feature. Here, Bayesianism is the view that probability is
“a measure of one’s degree of belief on whether a given phenomenon will tke place.
QBism follows this view and derives quantum probability (Born rule) as a measure
of the measurer’s degree of belief.
 The research question I raise here is whether QBism can derives other relevant
concepts in probability theory such as conditional probability and relative frequency.
The answer is expected to be affirmative, since in Bayesian approach to classical
probability theory, Italian mathematician Bruno de Finetti actually derived these.
 In this talk, I would like to brief review of Bruno de Finetti’s work and QBism,
followed by my recent work which derives conditional probability and relative frequency
along the line of the thought of Bruno de Finetti’s works and QBism.


日時:2022年6月30日13:15-

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: 浅井 詩緒乃(奈良女子大学)

題目: Transition between vacuum and finite-density states in the Bose–Hubbard model with spatially inhomogeneous dissipation

概要: Recent technological advances in cold-atom experiments enable us to analyze open quantum many-body systems with widely controllable dissipation [1–3]. Previous theoretical studies have considered the dynamics of a Bose–Hubbard system with dissipation processes that tend to lock the phase difference of the particle field between nearest-neighboring sites. It has been shown that steady states reached after long time evolution exhibit a transition between a Bose condensed state and a non-condensed state when the dissipation strength is varied [4,5]. Such transition phenomena in Bose–Hubbard systems with dissipation have recently attracted much attention.

In this work, we analyze the dynamics of the infinite-dimensional Bose–Hubbard model with spatially inhomogeneous dissipation by solving the Lindblad master equation with use of the Gutzwiller variational method [6]. We consider dissipation processes that correspond to inelastic light scattering in the case of Bose gases in optical lattices. We assume that the dissipation is applied to half of the lattice sites in a spatially alternating manner. We focus on steady states at which the system arrives after long time evolution. We find that when the average particle density is varied, the steady state exhibits a transition between a state in which the sites without dissipation are vacuum and one containing a finite number of particles at those sites. We also discuss whether this transition can occur also at finite dimensions on the basis of numerical analyses using the cluster mean field approximation.

 

1 A. Daley, Adv. Phys. 63, 77 (2014).

2 Y. S. Patil, S. Chakram, and M. Vengalattore, Phys. Rev. Lett. 115, 140402 (2015).

3 T. Tomita, S. Nakajima, I. Danshita, Y. Takasu, and Y. Takahashi, Sci. Adv. 3, e1701513 (2017).

4 S. Diehl, A. Tomadin, A. Micheli, R. Fazio, and P. Zoller, Phys. Rev. Lett. 105, 015702 (2010).

5 A. Tomadin, S. Diehl, and P. Zoller, Phys. Rev. A 83, 013611 (2011).

6 S. Asai, S. Goto, and I. Danshita, Prog. Theor. Exp. Phys. 2022, 033I01 (2022).


日時:2022年7月6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: 金子 隆威(量子多体)

題目: Renyi entropy after a quantum quench starting from insulating states in a free boson system

概要: The concept of entanglement is indispensable for understanding quantum many-body physics these days. The entanglement entropy quantifies the degree of entanglement and is a valuable probe for characterizing states of quantum many-body systems. The protocol for measuring the Renyi entropy first proposed in 2012 [1,2] has further stimulated experimental and theoretical research on real-time dynamics of quantum many-body lattice systems. Yet, studies of the entanglement growth during the quench dynamics for (soft-core) bosons are very few, even for a quench to a noninteracting region [3,4].Here we focus on a one-dimensional free boson system and investigate the time-dependent Renyi entanglement entropy after a quantum quench starting from the Mott insulating and charge-density-wave states. We show that the second Renyi entropy is the negative of the logarithm of the permanent of a matrix consisting of time-dependent single-particle correlation functions. We obtain rigorous conditions for satisfying the volume-law entanglement using the permanent inequality [5]. We also successfully calculate the time evolution of the entropy in unprecedentedly large systems by brute-force computations of the permanent. We discuss possible applications of our findings to the real-time dynamics of noninteracting bosonic systems.

[1] A. J. Daley et al., Phys. Rev. Lett. 109, 020505 (2012).
[2] D. A. Abanin and E. Demler, Phys. Rev. Lett. 109, 020504 (2012).
[3] M. Cramer et al., Phys. Rev. Lett. 100, 030602 (2008).
[4] A. Flesch et al., Phys. Rev. A 78, 033608 (2008).
[5] R. Berkowitz and P. Devlin, Isr. J. Math. 224, 437 (2018).

日時:2022年7月22日10:45-

教室: 31号館5階502教室 + Zoomでのオンライン配信

発表者: 長尾 一馬(理化学研究所)

題目: Squeezed coherent state path integral methods for fermions

概要: Spontaneous symmetry broken phases, such as Bose-Einstein condensates of dilute bosonic atoms and superconductors of electrons, are ubiquitous and fundamental quantum phases in condensed matter physics, AMO physics, and high-energy physics. In recent years, the Higgs mode of fermionic superconductor/superfluid systems, which is an emergent amplitude excitation mode of the order parameter field, has been extensively explored in many experiments including pump-probe experiments for BCS-type superconductors [1] and a radiofrequency spectroscopy experiment for ultracold two-component Fermi gases [2].

 In this work, towards establishing a versatile framework to describe those experimental systems, we develop a new path integral representation of fermionic superfluid systems that gives a unified description of the fermionic quasiparticle excitations and the bosonic collective excitations of the order parameter fluctuations. Our approach gives a direct fermionic generalization of the squeezed-coherent-state path integral method for bosons [3]. In this talk, we apply this formalism to BCS superconductors and discuss that a generalized Lagrangian defined for a squeezed fermionic coherent state shows a gapped excitation branch corresponding to the Higgs excitation mode and reproduces the well-known BCS quasiparticle gapped spectrum in a single framework [4]. Additionally, I will also talk about an ongoing project on an application of the method to contact interacting Fermi gas systems [5]. In this work, we analyze the generalized Lagrangian in terms of the momentum-shell renormalization group method and present a new finding on the ground state BCS- BEC crossover phenomenon of the dilute Fermi gas systems.

[1] R. Shimano and N. Tsuji, Annu. Rev. Condens. Matter Phys. 11, 103 (2020).

[2] A. Behrle et al., Nat. Phys. 14, 781 (2018).
[3] I. M. H. Seifie, V. P. Singh, and L. Mathey, Phys. Rev. A 100, 013602 (2019).

[4] K. Nagao, D. Li, and L. Mathey, arXiv:2102.03113.

[5] K. Nagao and L. Mathey, in preparation.

公式には近畿大学大学院総合理工学研究科学際セミナーとして開催。


日時:2022年7月27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: 井上 直人(一般相対論・宇宙論研究室)

題目: Ryu-Takayanagi 公式と量子情報

概要: ブラックホールは星が重力崩壊したことによって生じた天体であることを考慮すると、星内部の情報はブラックホールの内部に隠されてしまっていると考えることができる。しかし Bekenstein-Hawking 公式によって示されたブラックホールの隠し持つ情報量(エントロピー)は熱力学のエントロピーと異なる性質を持つものであった。そこで ’t Hooft と Susskind は、この違いが次元の違いから生じていると考えホログラフィ原理を提唱し、1997 年には Maldacena が「d+1 次元の反ド・ジッター空間の量子重力理論は、その境界上における d 次元共形場理論と等価である」というAdS/CFT 対応を発表した。
 AdS/CFT 対応の発展として Ryu と Takayanagi は時刻一定面において (d+2) 次元の AdS時空における d 次元の極小曲面の面積から、(d+1) 次元共形場理論におけるエンタングルメント・エントロピーが導出できると提唱した [1][2]。

     本発表では Ryu-Takayanagi の論文 [1][2] の解説を行う。

 

1. S. Ryu and T. Takayanagi, PhysicalL Review Letters, 96,
181602, 2006
2. S. Ryu and T. Takayanagi, Journal of High Energy Physics,
8, 45, 2006


日時:2022年8月5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: Andreas Thomasen(QunaSys)

題目: Contemporary approaches to variational quantum algorithms for quantum chemistry

概要:Variational quantum algorithms were initially proposed as enabling the first use-cases of quantum computers, namely determination of ground state properties of molecules, with the variational quantum eigensolver being the most familiar and well-known among them. There are signs that such algorithms have inherent short-comings that in the short term will render them unable to produce useful results on noisy intermediate-scale quantum (NISQ) devices due to noise and even for high-fidelity architectures may not converge due to the occurrence of barren plateaus in the parameter search space. In this seminar we give an overview of these problems and discuss recent approaches to mitigating and potentially overcoming them.


日時:2022年9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: 関野 裕太(理化学研究所)

題目: Optical spin conductivity in ultracold atomic gases

概要: In solid-state physics, measurement of optical conductivity, which is conductivity of an alternating electric current obtained in an optical way, plays crucial roles to understand various exotic electron systems such as high-Tc superconductors, non-Fermi liquids, and Dirac electrons [1,2,3]. Similarly, conductivity of an alternating spin current (optical spin conductivity) is expected to be a useful probe to investigate various quantum many-body systems from spin degrees of freedom.

 In this presentation, we report our proposal to measure optical spin conductivity of ultracold atomic gases [4]. In this proposal, an alternating spin current is induced by using a time-dependent gradient of magnetic field or the optical Stern-Gerlach effect. The generated current is measured by observing dynamics of spin density profile.

 To demonstrate what information the frequency dependence of the spin conductivity can capture, we computed the conductivity of several cold-atomic systems; a spin-1/2 s-wave Fermi superfluid, spin-1 Bose Einstein condensate, Tomonaga-Luttinger liquid, and topological Fermi superfluid [4,5]. In these systems, nontrivial spin excitations result in the frequency dependence of the spin conductivity quite different from that of the conventional Drude conductivity.

References:

[1] C. C. Homes et al., Phys. Rev. Lett. 71, 1645 (1993).

[2] Y. S. Lee et al., Phys. Rev. B 66, 041104(R) (2002).

[3] R. R. Nair et al, Science 320, 1308 (2008); K. F. Mak et al., Phys. Rev. Lett. 101, 196405 (2008).

[4] Y. Sekino, H. Tajima, and S. Uchino, arXiv:2103.02418.

[5] H. Tajima, Y. Sekino, and S. Uchino, Phys. Rev. B 105, 064508 (2022).

公式には近畿大学大学院総合理工学研究科学際セミナーとして開催。


日時:2022年10月5日9:00-

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1: 藪内 雄大(量子多体)

題目:2成分Bose-Hubbard 模型における相対論的量子液滴

概要:原子気体の冷却技術及びトラップ技術の進歩により高い制御性を獲得した冷却原子系は、量子多体現象の研究に理想的な舞台を提供している。例えば、Bose気体系におけるBose-Einstein 凝縮[1,2]や、光格子を用いて引き起こされる超流動からMott絶縁体への量子相転移[3]の実現は物性物理学の研究に大きなインパクトを与えた。比較的近年に実現されて注目を集めている新奇な多体現象の一つに量子液滴[4]がある。量子液滴はBose-Einstein凝縮にあるBose気体から形成される超流動体であり、またその(粒子数)密度が空間的に有限な範囲で一定に保たれるため、その名の通り液滴のような振る舞いをし、注目されている。量子液滴が注目されている理由として、非常に希薄であることと、液滴の形成に必要な自己束縛力の起源が非自明であること[5]、が主に挙げられる。ごく最近の研究で、斥力作用のみを持つ2成分Bose気体において光格子を用いて量子液滴を形成、安定化させる機構がMachida 氏らによって提案された。光格子中の2成分Bose気体における超流動からMott絶縁体への量子相転移は、1格子サイト当たりの粒子数が偶数であるとき、一次転移になりうる。この一次転移点上では、超流動秩序変数の時間発展を記述するGinzburg-Landau方程式に実効的に引力として作用する項が現れ、これにより液滴に対応する解が存在する[6]。

 本研究では、この系の液滴状態に関してさらに考察を行う。Bose-Hubbard模型のパラメータをMott相の先端(一次転移点)にとると、Ginzburg-Landau方程式は、particle-hole対称性を持つ非線形Klein-Gordon方程式の形をとる。これはローレンツ不変な方程式であるためこれにより記述される量子液滴は相対論的な振る舞いをする。この相対論的な量子液滴がみせる新たな性質、振る舞いを研究するのが本研究の目的である。本発表では、Bose-Hubbard模型の大分配関数を出発点としたGinzburg-Landau方程式の導出と、そこから得られる相対論的超流動体の励起スペクトルの性質について簡単なおさらいをする。

 

参考文献

[1] M. H. Anderson et al., Science 269, 198 (1995)

[2] K. B. Davis et al., Phys. Rev. Lett. 75, 3969 (1995)

[3] M. Greiner et al., Nature 415, 39 (2002)

[4] C. R. Cabrera et al., Science 359, 301-304 (2018)

[5] D. S. Petrov, Phys. Rev. Lett. 115, 155302 (2015)

[6] Y. Machida et al., Phys.Rev. A 105, L031301 (2022)


発表者2: 木佐貫 晋吾(量子多体)

題目: ラビ結合とスピン依存ホッピングを持つFermi-Hubbard modelにおける超伝導状態の臨界速度の理論解析に向けて

概要:極低温に冷却された二成分Fermi気体系では、Feshbach共鳴を利用してs波散乱長を調整することで、原子間の引力が弱いBCS状態から原子間の引力が強いBEC状態へ状態が推移するBCS-BECクロスオーバー現象が起こることが知られている[1]。この現象の実現以来、この系の超流動状態に関する様々な現象の研究が盛んに行われている。より近年の研究では、光格子というレーザーによって作られる原子にとっての周期ポテンシャルを用いてより多くの要素を制御することで、p波もしくはd波のクーパー対を持つ超流動状態や新奇な超流動発現機構の探索が進められている。
 本研究では、光格子中の2成分Fermi気体において、成分間にラビ結合があり、ホッピングが成分に依存する場合を考える。そのような状況は状態依存する光格子中のYb原子気体系で実現され、注目を集めている[2,3]。先行研究では、ラビ結合とスピン依存ホッピングを持つFermi-Hubbard 模型[4]を解析し、空間一次元におけるこの系の基底状態相図を描いた[5]。特に一方の成分のホッピングがゼロで超流動性が一般的には抑制されている場合に焦点を当て、ラビ結合の強さを変化させたときに超流動性が共鳴的に増進されるパラメータ領域を発見した[5]。本研究では、この新奇な超流動発現機構をより詳細に理解するために、平均場理論を用いて、空間三次元における同じ模型の超流動臨界速度を計算することを目的とする。冷却気体系において、実験で測りやすい超流動性の証拠であるという意味で、臨界速度は重要な物理量である。本発表では、そのための準備として連続系におけるBCS-BECクロスオーバーの平均場理論をおさらいし、それを用いて臨界速度を計算した先行研究[6]を紹介する。さらに、ラビ結合とスピン依存ホッピングを持つFermi-Hubbard模型の各項の意味を説明し、今後の方針について議論する。

 

[1] 大橋洋士, 物性研究, 85-2, 159-250 (2008).
[2] L. Riegger et al., Phys. Rev. Lett. 120, 143601 (2018).
[3] K. Ono et al., Nat. Commun. 12, 6724 (2021).
[4] W. Vincent Liu, F. Wilczek, and P. Zoller, Phys. Rev. A 70, 033603 (2004).
[5] M. Mikkelsen et al., arXiv:2203.04523v1.
[6] H. Yamamura and D. Yamamoto, J. Phys. Soc. Jpn. 84, 044003 (2015).


発表者3: 百合 巧(量子多体)

題目: 量子スピン系のEfimov効果の数値的研究に向けて

概要:1970年にEfimovは、核物理学の分野において特異な3体束縛状態が存在することを提唱し[1]、現在この状態はEfimov状態と呼ばれている。冷却原子系の実験において、2000年頃に粒子間の散乱長を幅広くコントロールできる技術が確立し、2006年にGrimmらのグループがその技術を用いて初めてEfimov状態を観測した[2]。上記のような核物理と原子物理の系に加えて、近年では量子スピン系においてもEfimov効果が現れることが提唱された[3]。冷却原子系においてはEfimov状態の形成に伴っておこる3体損失のために系自体が崩壊してしまうのに対して、量子スピン系においてはそのような崩壊がおこらない。この性質はEfimov状態に起因する新奇な多体現象を探索する上で大きな利点である。

 本研究では、量子スピン系に発現するEfimov効果を理解し、そこに現れる新奇量子現象を探求することを目的とする。本発表では、そのために現在まで学習したこととして、初めに、NaidonとEndoの総説論文[4]に基づいて、Efimov状態について理論的に説明する。具体的には、量子2体問題を簡単におさらいした後に、量子3体問題の理論的手法に基づいてEfimov状態の束縛エネルギーの計算を再現する。さらに、非等方Heisenberg模型の強磁性相においてマグノン励起がEfimov状態を発現するという先行研究[3]の結果を説明する。

 

[1] V. Efimov Yad. Fiz., 12, 1080-1091 (1970)

[2] R. Grimm et al., Nature 440 (2006) 315

[3] Y. Nishida et al., Nature Physics 9, 93-97 (2013)

[4] P. Naidon and S. Endo, Rep. Prog. Phys. 80, 056001 (2017)


日時:2022年10月12日9:00-

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1: 武上 響生(量子多体)

題目: ガウス型ポテンシャル障壁に対するBogoliubov励起の異常トンネル効果の数値解析に向けて

概要: 1995年に冷却Bose原子気体からなるBose-Einstein凝縮体(BEC)が初めて実現されて以来[1]、巨視的スケールの波動性に起因したBECの物性が活発に研究されてきた。例えば興味深い物性として超流動、量子渦、Josephson効果、異常トンネル効果などが挙げられる。本研究では、その中でもBogoliubov励起の異常トンネル効果に注目する。異常トンネル効果とは、希薄ボース気体のBECの素励起であるBogoliubov励起がポテンシャル障壁に入射した際に低エネルギー領域では完全透過するという現象である[2,3]。最初の予言から20年以上が経過しているにも関わらず、異常トンネル効果はいまだ実験で観測されていない。

 そこで我々は、クラウド型遠隔実験装置Albert[4]を用いてBogoliubov励起の異常トンネル効果を観測することを計画している。実験で異常トンネル効果を同定する上で、実験に用いられるガウス型のポテンシャル障壁に対するBogoliubov励起の透過確率の理論値が比較対象として必要である。本研究では、Bogoliubov励起を記述するBogoliubov方程式を数値的に解くことで、この場合の透過確率の理論値を与えることを目的とする。本発表ではそのための準備として、BECの波動関数を記述するGross-Pitaevskii(GP)方程式を説明し、GP方程式からのBogoliubov方程式の導出をおさらいする。次に、長方形型ポテンシャルの場合[3]を例にとり、異常トンネル効果の重要な性質を解説する。さらに、透過確率を数値的に求めるために用いる有限要素法について説明し、練習問題として一次元Schrödinger方程式のトンネル問題の透過確率を計算する。

 

[1] L. P. Pitaevskii and S. Stringari, Bose–Einstein Condensation., (Oxford University Press, Oxford, 2003)

[2] D. L. Kovrizhin, Phys. Lett. A 46 , 392 (2001)

[3] Yu. Kagan, D. L. Kovrizhin, and L. A. Maksimov, Phys. Rev. Lett. 90, 130402 (2003).

[4] https://bec.coldquantaapis.com


発表者2: 安藤 京介(物性理論)

題目: 機械学習による二次元正方格子XY模型の解析

概要:昨今の画像認識や画像分類、自然言語処理における人工ニューラルネットワークの目覚ましい発展は多くの科学分野に影響を与え、この技術をどのような問題に適用することで新たな発見が得られるかの探索が始まった。古典統計物理学の分野では対称性の破れた相を同定するために機械学習アルゴリズムが導入され[1-3]、そのうちいくつかのケースではニューラルネットワークが秩序パラメータやその他の熱力学的パラメータを学習できることが示されている[1,3]。従来の相転移について機械学習技術を適用できたことにより、非従来型の相転移に対してもこのアルゴリズムが適用できるか問うことは自然なことである。そのような系の例としてKosterlitz-Thouless転移(KT転移)を示す2次元XY模型が存在する[4]。

 本研究ではよく理解されている二次元正方格子XY模型について、機械学習アルゴリズムが既知の結果を再現できるかどうかを確認することを目的とする。本発表ではモンテカルロシミュレーションによって得られたスピン配位が既知の相関関数のふるまいを再現していることを確認する。

[1] J. Carrasquilla and R. G. Melko, Nat. Phys. 13, 431 (2017).

[2] E. P. L. van Nieuwenburg, Y.-H. Liu, and S. D. Huber, Nat. Phys.13, 435 (2017).

[3] S. J. Wetzel and M. Scherzer, Phys. Rev. B 96, 184410 (2017).

[4] A. Krizhevsky, I. Sutskever, and G. Hinton, Commun. ACM 60, 84 (2017).


発表者3: 柳川 颯斗(量子多体)

題目: 拡張Bose-Hubbard模型における古典カオスの数値解析に向けて

概要:カオスとは、非線形な力学系において、初期状態の僅かな変化が時間発展とともに、指数関数的に増大するというものである。その度合いはリアプノフ指数によって測られる。また、量子力学系に現れるカオスは量子カオスと呼ばれている。カオスと熱平衡化には、どちらも系の長時間発展後の性質は初期状態によらないという共通点があり、そのため、カオスは多体系の熱平衡化を理解する上で重要な概念だと考えられている。非線形力学系における熱平衡化の研究は、1955年のFermi、Pasta、Ulamによる一次元非線形連成バネ振り子の研究[1]に遡る。21世紀に入ってから、冷却原子系を用いることによりエネルギーの保存する多体系の熱平衡化に関する実験が大きく進展してきたため、近年の物性物理学の研究ではカオスに関する興味が再燃している[2]。

 本研究では、最近Shenらによって提唱された「量子臨界点でカオス性が極大となる」という仮説[3]に注目する。Shenらはこの仮説を検証するための具体的な例として、1次元Bose-Hubbard模型における超流動相とMott絶縁体相の間の量子相転移を考えた。しかしながら、この相転移はBerezinskii-Kosterlitz-Thouless転移であるせいで、数値計算が可能な小さいサイズでは量子臨界的な性質を示さないため、この仮説の検証に最適だとは言い難い。そこで本研究では、高次元の拡張Bose-Hubbard模型における超流動相と超固体相の間の量子相転移[4]を例にとり、その古典極限においてカオス性が極大となるかどうかを調べる。本発表ではそのための準備として1次元離散Schrödinger方程式で記述される系に関する先行研究[5]を紹介し、その主たる内容であるリアプノフ指数の数値計算の再現に向けた進捗を報告する。そして、拡張Bose-Hubbard模型について勉強をしたこととして、Gross-Pitaevskii平均場近似の範囲での超流動・超固体量子相転移の記述を説明する[5]。

 

[1] E. Fermi, J. Pasta and S. Ulam, Los Alamos Report No. LA-1940 (1955)

[2] L. D’Alessio et al., Adv. Phys. 65, 239 (2016)

[3] Huitao Shen et al., Phys. Rev. B 96, 054503 (2017)

[4] I. Danshita and C. A. R. Sa de Melo, Phys. Rev. Lett. 103, 225301 (2009)

[5] A. C. Cassidy et al., Phys. Rev. Lett. 102, 025302 (2009)


発表者4: 西田 翔(量子制御)

題目: 学生実験用NMR量子コンピュータと量子アルゴリズムの実装

概要: 量子コンピュータを学生実験として実験したい。そのために、3年生の学生実験で使用している卓上NMRを利用したいが、問題が二つ発生する。1つ目は、卓上NMRの小さい磁場ではSN比が悪くなること。2つ目は永久磁石を使うため、温度変化に弱いことである。この2つの問題を解決するために、複数の測定を平均する「積算」と温度変化に追随するプログラムをpythonで組み、NMR量子コンピュータ用の環境を構築している。報告では、私が構築したシステムと実装する量子アルゴリズムの概要、相互作用によって分裂したピークの観測例を報告する。


日時: 2022年10月26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: 金子 隆威(量子多体)

題目: Tensor-network study of the SU(4) Heisenberg model on a plaquette lattice

概要: Lattice models with SU(N>2) degrees of freedom have recently attracted much interest because of possible emergent novel quantum states that do not appear in the Hubbard and Heisenberg models with SU(2) degrees of freedom. Experimental systems to realize these models include ultracold atomic gases in optical lattices [1-6] and antiferromagnets with intertwined spin and orbital degrees of freedom [7,8]. Recently, Ozawa et al. have reported that Pomeranchuk cooling facilitates the observation of antiferromagnetic correlations in SU(N) systems for sufficiently large N [6]. Miyazaki et al. have investigated the ground state of the relevant SU(4) Heisenberg model on a plaquette lattice by the cluster mean-field and spin-wave approximations [9,10]. They have found the SU(4) singlet ground state in the strongly anisotropic regime. On the other hand, the prior tensor-network study suggested that the ground state should exhibit the coexisting Neel and the valence bond crystal (VBC) order in the nearly isotropic region [5]. This observation implies the presence of the phase transition in the intermediate anisotropy region. To investigate the ground states of the SU(4) Heisenberg model on a plaquette lattice more precisely, we use the two-dimensional tensor-network method based on the infinite projected entangled pair states [11,12]. We successfully reproduce the SU(4) singlet ground state in the strongly anisotropic limit and the Neel-VBC coexisting ground state in the nearly isotropic limit. We will examine the location of the phase transition point and discuss the relevance of our results to future experiments.

[1] C. Wu et al., Phys. Rev. Lett. 91, 186402 (2003).
[2] C. Honerkamp and W. Hofstetter, Phys. Rev. Lett. 92, 170403 (2004).
[3] M. A. Cazalilla et al., New J. Phys. 11, 103033 (2009).
[4] A. V. Gorshkov, Nat. Phys. 6, 289 (2010).
[5] P. Corboz et al., Phys. Rev. Lett. 107, 215301 (2011).
[6] H. Ozawa et al., Phys. Rev. Lett. 121, 225303 (2018).
[7] P. Corboz et al., Phys. Rev. X 2, 041013 (2012).
[8] S. Nakatsuji et al., Science 336, 559 (2012).
[9] Y. Miyazaki et al., AIP Advances 11, 025202 (2021).
[10] Y. Miyazaki et al., J. Phys. Soc. Jpn. 91, 073702 (2022).
[11] T. Nishino et al., Prog. Theo. Phys. 105, 409 (2001).
[12] F. Verstraete and J. I. Cirac, arXiv:cond-mat/0407066.


日時: 2022年11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: 木屋 晴貴(量子制御)

題目: Search for the shortest composite quantum gate 

概要: One of the largest problems in realizing a quantum computer is inevitable systematic device errors. We consider two types of systematic errors in single-qubit control: Pulse Length Error (PLE) and Off-Resonance Error (ORE). A composite quantum gate (CQG) is a method in order to compensate for such systematic errors [1]. CQGs use the redundancy obtained by decomposing the target operation into multiple ones. The gate fidelity and operation time are important to evaluate the performance of CQG. A shorter operation time is better for reducing the effects of decoherence and noises. There is, however, not much discussion on the lower bound of operation time [2]. We are interested in this bound.  

In order to obtain a hint of the bound, we exhaustively generate high fidelity gate combinations and examine their operation times. This survey suggests the existence of nontrivial bounds on operation time for CQGs.

参考文献

[1] Malcolm H. Levitt, Ray Freeman, J. Magn. Reson.33,473 (1979)

[2] Shingo Kukita, Haruki Kiya, Yasushi Kondo, Phys. Rev. A 106, 042613 (2022)


日時: 2022年11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: 數田 裕紀(量子多体)

題目: Quantum simulation of non-ergodic behavior of the Bose-Hubbard model with a trapping potential

概要: The development of information technology in last few decades has enabled us to do many kinds of things on the cloud via internet. Such a cloud system is advantageous in the sense that it can be used at any time, at any place, and by anyone in the world. Of our particular interest are cloud services of advanced quantum devices, such as IBMQ (Quantum computer equipped with superconducting quantum qubit provided by IBM) [1] and Albert (Platform to create and control with Bose-Einstein Condensate (BEC) provided by ColdQaunta) [2], which have become open to the public rather recently. In Japan, the Quantum Optics group at Kyoto University plans to launch a cloud service of their quantum simulator based on ultracold atoms in optical lattices. We support its development as users of a preliminary version of the service. Since optical-lattice quantum simulators are suitable for studying non-equilibrium dynamics of quantum many-body systems [3], which is difficult to access in numerical calculations on classical computers, we aim to analyze non-ergodic dynamics due to the Hilbert-space fragmentation in the 1D Bose-Hubbard system by using the quantum simulator remotely [4].

In this presentation, I focus on the time evolution of the initial state in which each odd-numbered site is doubly occupied and each even-numbered site is empty. I will first present some preliminary results of our attempts for observing the non-ergodic behavior, which have not yet been succeeded, with the use of the remote quantum simulator. I will explain several possible reasons of the failure in the observation, among which the effect of the interchain hopping seems dominant. In order to theoretically analyze the effect of the interchain hopping, we try to numerically simulate dynamics of the Bose-Hubbard model on a two-leg ladder lattice. For this purpose, I will review how to apply the time-evolving block decimation method to the two-leg ladder geometry [5].

 

[1] https://quantum-computing.ibm.com/

[2] https://albert.coldquanta.com/

[3] F. Schäfer, T. Fukuhara, S. Sugawa, Y. Takasu, and Y. Takahashi, Nat. Rev. Phys. 2, 411 (2020).

[4] M. Kunimi and I. Danshita, Phys. Rev. A 104, 043322 (2021).

[5] I. Danshita, J. E. Williams, C. A. R. Sá de Melo, and C. W. Clark, Phys. Rev. A 76, 043606 (2007).


日時: 2022年11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: 小久保 治哉(物性理論)

題目: Size dependence of a plate obstacle for vortex generation and dynamics in superfluid wake

概要:  In classical hydrodynamics, a wake appearing behind a moving obstacle is characterized by a Reynolds number, the ratio of inertial forces to viscous forces. The Reynolds number is a quantity proportional to the characteristic length of the obstacle. As the Reynolds number increases, an unstable flow appears.
 A wake appears by considering obstacles using lasers in cold atom systems [1]. Furthermore, the Reynolds number in superfluids has also been suggested [2]. The Superfluid Reynolds number includes the critical velocity of the vortex generation and is not the ratio of inertial to viscous force as in the classical system.
 In this study, we study the size dependence of a plate obstacle in the Reynolds number by considering a wake in a 2D BEC system with a plate obstacle. Plate obstacle is very useful because it doesn’t need to consider density decay caused by gradient around the obstacle and size of the obstacle can be characterized only by the width of the plate. In this talk, we introduce the dynamics of the wake induced by the plate obstacle when its width is changed and discuss the future outlook.
[1]T.W. Neely, et al. Phys. Rev. Lett. 104, 160401 (2010)
[2]M.T. Reeves, et al. Phys. Rev. Lett. 114, 155302 (2015)


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