Time and Date: 10:45-, April 19, 2023

Room: Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker: Kokubo, Haruya (CMT)

Title: Size dependence of the critical velocity for quantum vortex formation by the superfluid wake with a plate obstacle

Abstract: The wake generated by an object moving through a fluid depends on the size and velocity of the obstacle and is associated with various physical phenomena such as vortex formation and turbulent transition. The wake has been studied experimentally and theoretically in superfluids. In cold atomic systems, optical potentials have been used in superfluid wake experiments [1-2]. In numerical simulations [3-4], Gaussian potential is often assumed as obstacles. However, the dynamics of the wake and the critical velocity for vortex formation depend on the specifics of the obstacle potential, complicating the universal discussion. Plate obstacle is comparatively easy to handle both theoretically and numerically. The size of the obstacle can be characterized only by the width of the plate, making it easy to study its effects.In this study, the size dependence of the critical velocity for quantum vortex formulation in the wake of a 2D Bose-Einstein condensate with the plate obstacle is investigated.

[1]Woo Jin Kwon, Joon Hyun Kim, Sang Won Seo, and Y. Shin, Phys. Rev. Lett. 117, 245301 (2016)

[2]Younghoon Lim, et al, New J. Phys. 24, 083020 (2022)

[3]Kazuki Sasaki, Naoya Suzuki, and Hiroki Saito, Phys. Rev. Lett. 104, 150404 (2010)

[4]M. T. Reeves, et al, Phys. Rev. Lett. 114, 155302 (2015)

Time and Date: 10:45-, April 26, 2023

Room: Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker: Kasamatsu, Kenichi (CMT)

Title: Recent understanding and problems on vortex dynamics in binary Bose-Einstein condensates

Abstract: I review some recent progress and remaining problems on the properties of quantized vortices in two-component (binary) Bose-Einstein condensates (BECs). A vortex in binary BECs takes a rich variety of structures, forming a half-quantized vortex, as a result of the presence of multiple order parameters. This vortex exhibits nontrivial structure and dynamics when we consider the system of several or lots of vortices. In this talk, we will discuss (i) Interactions and dynamics of two separated vortices [1,2], (ii) Structures of vortex lattices in a rotating potential [3], (iii) Quantum turbulence by stirring of a localized potential [4].

[1] K. Kasamatsu, M. Eto, and M. Nitta, Phys. Rev. A, 93, 013615 (2016).

[2] J. Han, K. Kasamatsu, and M. Tsubota, J. Phys. Soc. Jpn. 91, 024401 (2022).

[3] K. Kasamatsu, M. Tsubota and M. Ueda, Int. J. Mod. Phys. B 19, 1835 (2005).

[4] T. Mithun, K. Kasamatsu, B. Dey, and P. G. Kevrekidis, Phys. Rev. A 103, 023301 (2021).

Time and Date: 10:45-, May 10, 2023

Room: Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker: Kagamihara, Daichi (QMB)

Title: Classical simulation of non-Hermitian boson sampling dynamics using matrix product states

Abstract:

Sampling complexity means the difficulty of sampling from a distribution close to the desired (e.g., quantum system) probability distribution and is discussed in the context of quantum supremacy. A famous example where sampling complexity is often discussed is boson sampling [1], which consists of sampling from the distribution of identical bosons generated by a linear interferometer. An interferometer is characterized by a unitary matrix. It is shown that boson sampling is hard by classical computers for random unitary matrices. Recently, by considering the sampling problem of identical bosons time-evolved by a quadratic Hamiltonian from a product state, a transition from the state where sampling can be done easily to one where it is difficult has been proposed [2]. 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 [3].

In the previous QPT seminar, we studied the Rényi entanglement entropy in the non-Hermitian boson sampling dynamics discussed in Ref. [3] and found that low-entangled states appear in some parameter regions. In this talk, we consider a classical simulation using matrix product states (MPS) with the expectation that low-entangled states would be well described using MPS and therefore an efficient classical simulation of the sampling would be possible. We investigate the dynamics of the bond dimension, which corresponds to the number of Schmidt states kept in MPS, because it is closely related to the efficiency of sampling [4]. Our results clarify the region where an efficient classical simulation can be performed using MPS.

[1] S. Aaronson and A. Arkhipov, in Proceedings of the Forty-Third Annual ACM Symposium on Theory of Computing (Association for Computing Machinery, New York, 2011), pp. 333–342.

[2] A. Deshpande, B. Fefferman, M. C. Tran, M. Foss-Feig, and A. V. Gorshkov, Phys. Rev. Lett. **121**, 030501 (2018).

[3] K. Mochizuki and R. Hamazaki, Phys. Rev. Res. **5**, 013177 (2023).

[4] H.-L. Huang, W.-S. Bao, and C. Guo, Phys. Rev. A **100**, 032305 (2019); C. Oh, K. Noh, B. Fefferman, and L. Jiang, Phys. Rev. A **104**, 022407 (2021).

Time and Date: 10:45-, May 24, 2023

Room: Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker: Polo, Juan（Quantum Research Center, Technology Innovation Institute）

Title: Fractionalization of the angular momentum in SU(N) atomtronic circuits

Abstract: Neutral atoms guided in ring-shaped atomtronic circuits present quantized values of the angular momentum per particle. Depending on the specific parameters characterizing the system (eg: nature of the particle statistics, interactions), the winding number present different quantizion properties. In this talk, I will showcase how such a phenomenon occurs in an atomtronic circuit with a quantum fluid consisting of strongly interacting N component fermions, the so-called SU(N) fermions. For repulsive interactions a specific emerging phenomenon of attraction from repulsion appears, providing a quantization with similar properties to the attracting bosonic case. For attractive interactions, the quantization is determined by the number of components N . The suggested implementation of our work is provided by cold atoms therefore, I will also present how the fractionalization of the winding number can be read-out experimentally.

Time and Date: 10:45-, May 31, 2023

Room: Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker: Danshita, Ippei (QMB)

Title: Correlation-spreading dynamics after a quantum quench in low-dimensional Ising models with transverse field

Abstract: Recent advances in controlling and manipulating Rydberg atoms trapped with an optical tweezer array have made it possible to utilize this platform as a quantum simulator of quantum Ising models, where dynamics of spin-spin correlations can be measured at a single-site resolution. Motivated by such experimental development, we theoretically study dynamics of spatial spreading of equal-time spin-spin correlations in the transverse-field Ising model subjected to a sudden change of the transverse field [1]. We assume that the initial state is a state polarized completely along the transverse direction, which is the ground state in the large-field limit. In one spatial dimension (1D), we use the exact analytical method and the linear spin-wave approximation (LSWA). We compare the outputs of the former with those of the latter in order to show that the latter can quantitatively capture the exact group velocity of the correlations as long as the transverse-field after the quench is sufficiently large compared to the spin-spin interaction. However, it fails to capture the detailed time dependence of the correlation functions. In 2D, with use of LSWA, we give a specific estimate of the group velocity in the large-field region. We also utilize the projected entangled pair states algorithm in order to provide quantitatively accurate time-evolution of the correlation functions for a relatively short time.

[1] R. Kaneko and I. Danshita, arXiv:2301.01407 [cond-mat.quant-gas].

Time and Date: 10:45-, June 14, 2023

Room: Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker: Mikkelsen, Mathias (QMB)

Title: Correlation spreading dynamics in SU(N) Fermi-Hubbard models

Abstract: For local Hamiltonians the propagation front of correlations is generally bounded by a light-cone like structure, where an initially localized correlation spreads to distant sites with a maximum velocity [1,2]. Loose bounds on this velocity can be found in a range of models using the Lieb-Robinson bound [1], but the actual speed is usually much slower. These concepts have been studied [3] and experimentally confirmed [2] quenching from a Mott-state to a finitely-interacting Bose-Hubbard (BH) model. These investigations have shown that a dip in the density-density correlations will propagate with a velocity which depends on the interaction strength U. For large U the physics can be effectively described in terms of doublon-holon excitations. Similarly SU(N) Fermi-Hubbard (FH) models will have excitations corresponding to doublon pairs with different flavors.

In this work we calculate SU(2)-SU(4) equal time density correlations and compare with the BH results using the time-evolving block decimation (based on the Suzuki-Trottter decomposition) method for tensor network representations of the system [4]. Preliminary results suggest that the propagation velocity is generally lower than the BH case, particularly for the standard SU(2) FH model, but increases with N asymptotically towards the BH case (in line with Lieb-Robinson bounds which are similar to bosons in the large N limit [1]). Our study is motivated by the successful experimental implementation of SU(N) Fermi-Hubbard models using Alkaline earth(-like) atoms [5,6]. While site-resolved measurement of dynamics remains an experimental challenge our investigation shows that potentially interesting physics can be found and provides a theoretical/numerical comparison for future experiments.

[1] Z. Wang and K. R.A. Hazzard, *PRX Quantum* **1**, 010303 (2020)

[2] M. Cheneau et al., *Nature* **481**, 484–487 (2012)

[3] P. Barmettler, D. Poletti, M. Cheneau and C. Kollath, *Phys.Rev. A ***85**, 053625 (2012)

[4] E. M. Stoudenmire and Steven R White, *New J. Phys.* **12, **055026 (2010)

[5] S. Taie, R. Yamazaki, S. Sugawa, and Y. Takahashi, *Nature Physics* **8**, 825 (2012).

[6] C. Hofrichter, et al. *Phys. Rev. X ***6**, 021030 (2016).

Time and Date: 10:45-, June 21, 2023

Room: Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker: Ando, Kyosuke (CMT)

Title: Machine learning analysis of XY models in two-dimensional square lattice

Abstract: The recent remarkable development of artificial neural networks in image recognition, image classification, and natural language processing has influenced many scientific fields, and the search for new discoveries by applying this technology to any problem has begun. In the field of classical statistical physics, machine learning algorithms were introduced to identify symmetry-broken phase [1-3], and in some of these cases neural networks were shown to be able to learn order parameters and other thermodynamic parameters [1,3]. Having been able to apply machine learning techniques to conventional phase transitions, it is natural to ask whether the algorithm can be applied to unconventional phase transitions. An example of such a system is a two-dimensional XY model that exhibits the Kosterlitz-Thouless transition (KT transition) [4].

The purpose of this study is to check whether machine learning algorithms can reproduce the known results for the well understood two-dimensional square lattice XY model. We confirmed that the spin configurations obtained by Monte Carlo simulations reproduce the known behavior of the correlation function, and trained the model with an all-associative neural network and a convolutional neural network. Using the trained model, we confirm the accuracy and size dependence of the transition temperature at which the KT transition occurs from spin configuration.

[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).

Time and Date: 13:15-, June 29, 2023

Room: Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker: Takegami, Hibiki (Kyoto University)

Title: Numerical analysis on anomalous tunneling of Bogoliubov excitations through a Gaussian potential barrier

Abstract: Elementary excitations of a weakly interacting Bose gas, which are referred to as Bogoliubov excitations [1], exhibit an anomalous tunneling property in that the transmission probability approaches unity as the excitation energy decreases towards zero [2,3]. Despite more than 20 years having passed since its first prediction [2], the anomalous tunneling has not been observed in experiments. We plan to observe the anomalous tunneling of Bogoliubov excitations using the cloud-based remote experimental apparatus of ultracold atomic Bose-Einstein condensates, called Albert [4]. For identifying the anomalous tunneling in experiments, it is necessary to have theoretical values for the transmission probability of Bogoliubov excitations through the Gaussian potential barrier used in the experiment, as a reference for comparison.

The purpose of this study is to provide theoretical values for the transmission probability in this case by numerically solving the Bogoliubov equation that describes the Bogoliubov excitations. First, we review the case of a rectangular potential, which allows for an approximate analytical expression of the transmission probability [3], as an example to illustrate the important properties of the anomalous tunneling. Next, we explain the finite element method that is used to numerically calculate the transmission probability [5]. We reproduce the calculation of the transmission probability for Bogoliubov excitations in a rectangular potential [3]. Finally, we calculate the transmission probability for a Gaussian potential and discuss optimal values of the barrier width and height for observing the anomalous tunneling with Albert.

[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

[5]計算物理Ⅱ, 夏目雄平および植田毅, 朝倉書店, 2002年.

Time and Date: 10:45-, July 5, 2023

Room: Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker: Kiya, Haruki (QC)

Title: Robust Single-Qubit Gates and Polygons on Sphere

Abstract: In single-qubit gates, two typical inevitable systematic errors, Pulse Length Error (PLE) and Off-Resonance Error (ORE) exist. PLE is often caused by a control field strength error, while ORE is by a resonance frequency calibration error of a qubit. These errors can be compensated by a method called Composite Quantum Gate (CQG): A single gate is replaced with a sequence of gates such that their errors are canceled with each other [1]. We discuss a symmetric CQG (sCQG) which is robust against PLE from the viewpoint of a corresponding polygon on a sphere.

We are interested in the gate times of CQGs [2] and Geometric Quantum Gates [3,4]. We hope that the graphical understanding of the PLE robust condition helps to find the shortest gate time and to deepen the understanding of CQGs.

[1] M. H. Levitt and R. Freedman, J. Magn. Reson.**33**,472(1979).

[2] S. Kukita, H. Kiya and Y. Kondo, submitted.

[3] Y. Kondo and M. Bando, J. Phys. Soc. Jpn.**80**,054002(2011).

[4] E.Sjöqvist, Int. J. Quantum Chem. 2015,**115**,1311-1326.

Time and Date: 10:45-, July 12, 2023

Room: Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker: Kazuta, Hironori (QMB)

Title: Quantum and classical simulations of non-ergodic behavior in a disorder-free Bose-Hubbard system

Abstract: In this work, following the protocol proposed in Ref. [1], we analyze non-ergodic dynamics in a one-dimensional Bose-Hubbard system by means of a quantum simulator built with ultracold Bose gases in optical lattices. Specifically, we analyze dynamics starting from two different initial states. In the first (second) state, one particle occupies (two particles occupy) each odd-numbered site while no particle occupies each even-numbered site. We observe the time evolution of the atomic density in order to show that in the case of the second state the atomic density is not relaxed to equilibrium even after long-time evolution, exhibiting non-ergodic behavior. In order to provide quantitative references to the quantum-simulation outputs, we present numerical simulations of the experiments with use of matrix product states.

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

Time and Date: 10:45-, July 19, 2023

Room: Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker: Kondo, Yasushi (QC)

Title: Composite Quantum Gates and Geometric Phase Gates

Abstract: After reviewing geometric phases, I shall introduce two examples of geometric phase gates applied (proposed) for Rydberg atoms (*PhysRevA.96.052316, **PhysRevResearch.2.043130). Then*, geometric properties of composite quantum gates are discussed (arXiv:2301.05627, JPSJ.80.054002). Some questionable points (= what I cannot understand) in the above papers are also discussed.

Time and date： 10:00-, October 4, 2023

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker 1： Ueda, Kenta（QMB）

Title：Only in Japanese

Abstract：Only in Japanese

Speaker 2： Watanabe, Genki（QMB）

Title：Only in Japanese

Abstract：Only in Japanese

Time and date： 9:30-, October 11, 2023

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker 1： Kimura, Ren（QMB）

Title：Only in Japanese

Abstract：Only in Japanese

Speaker 2： Morimoto, Rikuto（QMB）

Title：Only in Japanese

Abstract：Only in Japanese

Time and date： 10:45-, October 25, 2023

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker： Yabuuchi, Yoshihiro（Osaka Metropolitan University）

Title：Collective Excitations in Relativistic Quantum Droplets of Two-Component Bose-Hubbard Model

Abstract：Considerable progresses in cooling and controlling ultracold atoms have led to the realization of quantum droplets of two-component [1] or dipolar [2] Bose gases in continuum. More recently, Machida et al. have analyzed two-component Bose gases in optical lattices in order to show that a quantum droplet state can be stabilized at the first-order quantum phase transition between the superfluid and Mott-insulating states [3].

In this work, we focus on the fact that the Ginzburg-Landau (GL) equation describing the quantum droplets of the two-component lattice bosons acquires a Lorentz invariance near the effective particle-hole symmetric point [4]. We specifically investigate properties of some low energy excitations of such relativistic quantum droplets within the GL approximation. It is well known that the amplitude and phase fluctuations of the order parameter of a relativistic superfluid are perfectly decoupled [5]. In the case of amplitude modes, we find that the monopole and quadrupole modes are surface modes and that the dipole mode is the Nambu-Goldstone zero mode that emerges because the presence of the droplet spontaneously breaks the translational symmetry of space. In the case of phase modes, we find that monopole, dipole and quadrupole modes of phase are a bulk modes. We show that these results are in clear contrast with those of non-relativistic quantum droplets obtained previously in Ref. [6].

[1] C. R. Cabrera et al., Science 359, 301 (2018).

[2] I . Ferrier et al., Phys. Rev. Lett. 116, 215301 (2016).

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

[4] I. Danshita, D. Yamamoto, and Y. Kato, Phys. Rev. A 91, 013603 (2015).

[5] D. Pekker and C. M. Varma, Annu. Rev. Condens. Matter Phys. 6, 269 (2015).

[6] H. Hu and X.-J. Liu, Phys. Rev. A 102, 053303 (2020).

Time and date： 10:45-, November 8, 2023

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker： Miyazaki, Yuki（Aoyama Gakuin University）

Title：Evaluation of Quantum Entanglement via Permutationally Invariant Quantum State Tomography

Abstract：Quantum state tomography (QST), in which the density matrix of a quantum many-body system is reconstructed by the expectation values of a set of observables, is experimentally hard due to (i) the exponential increase of degrees of freedom with system size and, in the case of cold atomic systems in optical lattice, (ii) the practical problem of local quantization axis rotation. In a previous work [1], permutationally invariant (PI) QST was introduced as the reconstruction of the part of a density matrix that is invariant under permutations of lattice sites. It has been reported not only that PIQST can avoid the above issues, but also that the PI part of a density matrix can encode some important properties of the original density matrix [2]. In this work, we investigate the relation between the PI part of a density matrix and some entanglement measures and obtain some benchmarks for the relation.

[1] G. Tóth, W. Wieczorek, D. Gross, R. Krischek, C. Schwemmer, and H. Weinfurter, Phys. Rev. Lett. 105, 250403 (2010).

[2] T. Gao, F. Yan, and S.J. van Enk, Phys. Rev. Lett. 112, 180501 (2014).

Time and date： 10:45-, November 15, 2023

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker： Kagamihara, Daichi（QMB）

Title：Entanglement entropies in free boson systems

Abstract：Entanglement is one of the indispensable concepts in modern quantum physics. It describes non-classical and non-local correlations and is often quantified by entanglement entropies. While the von Neumann entanglement entropy is a standard measure of entanglement, its variant, Rényi entanglement entropy, was experimentally measured in highly controllable systems [1]. In this context, we theoretically studied the second-order Rényi entanglement entropy of free boson systems and revealed an interesting formula for it [2]. However, our formula is specific to the (second-order) Rényi entropy, and it is more desirable to access the von Neumann entanglement entropy directly.

In this talk, we generalize our result on the second Rényi entanglement entropy of free boson systems [2] to arbitrary order ones, including the von Neumann entanglement entropy. We also discuss possible applications of this result.

[1] R. Islam et al., Nature 528, 77 (2015); A. M. Kaufman et al., Science 353, 794 (2016); T. Brydges et al., Science 364, 260 (2019); D. Bluvstein et al., Nature 604, 451 (2022).

[2] D. Kagamihara, R. Kaneko, S. Yamashika, K. Sugiyama, R. Yoshii, S. Tsuchiya, I. Danshita, Phys. Rev. A 107, 033305 (2023).

Time and date： 10:45-, November 22, 2023

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker： Kokubo, Haruya（CMT）

Title：Critical velocity for quantized vortex formation in a superfluid with a plate-shaped obstacle

Abstract：Wake is the flow that occurs behind an obstacle moving through fluids, the dynamics of which is determined by the size and velocity of the obstacle, and is associated with various fluid phenomena such as vortex formation and turbulent transition. Wakes in superfluids have been studied both experimentally [1-2] and theoretically [3-4], and it has been shown that the critical velocity depends on the shape of the obstacle [5]. In numerical simulations, Gaussian potentials are often used to simulate an optical laser obstacle. However, it is difficult to measure the dependence of the critical velocity on the shape of the obstacle due to the unclear effects of the tail in the Gaussian potential. We consider the wake with a plate-shaped obstacle to evaluate the dependence of the critical velocity on the size of the obstacle. Plate-shaped obstacles have no shape profile other than width and can be easily measured both theoretically and numerically. In this talk, we describe the size dependence of the critical velocity by numerical simulations for a 2-dimensional Bose-Einstein condensate and present a method for quantitative evaluation of the critical velocity using the complex potential flow.

[1] Woo Jin Kwon, et al., Phys. Rev. Lett. 117, 245301 (2016)

[2] Younghoon Lim, et al., New J. Phys 24, 083020 (2022)

[3] Kazuki Sasaki, Naoya Suzuki, and Hiroki Saito, Phys. Rev. Lett 104, 150404 (2010)

[4] M. T. Reeves, et al., Phys. Rev. Lett. 114, 155302 (2015)

[5] Kwon, Woo Jin, et al., Phys. Rev. A 91, 053615 (2015)

Time and date： 10:45-, November 29, 2023

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker： Rammohan, Sidharth（QMB）

Title：Tailoring the Phonon Environment of Embedded Rydberg Aggregates

Abstract：State-of-the-art experiments can controllably create Rydberg atoms inside a Bose-Einstein condensate (BEC) [1]. The large Rydberg electron orbital volume contains many neutral atoms, resulting in electron-atom scattering events. The number of atoms within the orbit, and hence the Rydberg-BEC interaction, can be tuned by choice of principal quantum number or condensate density [1]. This makes the hybrid system a fascinating platform for quantum simulation. We studied the physics of the interaction and corresponding dynamics of single or multiple Rydberg atoms in two internal electronic states embedded inside a BEC, to assess their utility for controlled studies of decoherence and quantum simulations of excitation transport similar to photosynthetic light-harvesting.

We initially developed a theoretical framework to calculate the open quantum system input parameters like the bath correlation function and the spectral density, initially for a single Rydberg atom, possibly in two internal states with angular momentum quantum numbers l = 0 (|s⟩) and l = 1 (|p⟩) [2], in BEC and then for a chain of Rydberg atoms, forming an aggregate. The electron-atom contact interactions lead to Rydberg-BEC coupling, which creates Bogoliubov excitations (phonons) in the BEC.

Using this spin-boson model with the calculated parameters, we examine the decoherence dynamics of a Rydberg atom in a superposition of |s⟩ and |p⟩ states, resulting from the interaction with its condensate environment. Further, we investigated the emergence of the Non-Markovian features in the system in the presence of a microwave external drive of the Rydberg atom using a stochastic computational technique for Non-Markovian open quantum systems [3].

Finally, we extend this to the aggregate case, where one of the atoms in the aggregate is in the state |p⟩, while the rest are in the state |s⟩, resulting in excitation transport via dipole-dipole interaction [4]. We investigate the effects of Non-Markovinity and decoherence on the excitation transport based on an effective model described by a Holstein Hamiltonian, allowing us to set up the dynamics similar to those found in light-harvesting complexes, but at a different time and energy scales.

References:

- J. B. Balewski, A. T. Krupp, A. Gaj, D. Peter, H. P. Büchler, R. Löw, S.Hofferberth and T. Pfau; Nature 502 664 (2013).
- S. Rammohan, A. K. Chauhan, R. Nath, A. Eisfeld, and S. Wüster; Phys. Rev.A 103, 063307 (2021).
- S. Rammohan, S. Tiwari, A. Mishra, A. Pendse, A. K. Chauhan, R. Nath, A.Eisfeld, and S. Wüster; Phys. Rev. A 104, L060202 (2021).
- D. W. Schönleber, A. Eisfeld, M. Genkin, S. Whitlock and S. Wüster; Phys. Rev. Lett. 114 123005 (2015).

Time and date： 10:45-, December 6, 2023

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker： Kazuta, Hironori（QMB）

Title：Quantum simulation of nonergodic behavior in a disorder-free Bose-Hubbard system

Abstract：Thanks to their high controllability and near-perfect isolation from environment, cold-atom systems serve as a unique platform for studying non-equilibrium dynamics of isolated quantum systems. To understand how an isolated quantum system reaches thermal equilibrium through unitary time evolution, it is important to investigate mechanisms of nonergodic systems, such as integrability [1], many-body localization [2], quantum many-body scar states [3], and the Hilbert space fragmentation (HSF) [4]. A previous theoretical study [5] proposed that non-ergodic dynamics due to HSF originated from strong interparticle interactions can occur in a one-dimensional Bose-Hubbard system with weak trapping potentials when in the initial state of the dynamics the odd-numbered (even-numbered) sites are doubly occupied (empty). In this presentation, following the proposed protocol, we experimentally analyze this nonergodic dynamics by means of a quantum simulator built with ultracold Bose gases in optical lattices. To cross-check the quantum-simulation results, we numerically simulate the real-time evolution with use of matrix product states combined with the local density approximation.

[1] T. Kinoshita, T. Wenger, and D. S. Weiss, Nature **440** 900 (2006).

[2] J. Choi et al., Science **352**, 1547 (2016).

[3] H. Bernien, et al., Nature **551**, 579 (2017)

[4] S. Scherg et al., Nat. Comm. **12**, 4490 (2021)

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

Time and date： 10:45-, December 13, 2023

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker： Gietka, Karol（University of Innsbruck）

Title：Combining quantum with critical metrology, and temperature enhanced critical metrology

Abstract：In the talk, I will present two ways of improving quantum critical metrology. First relies on a combination of conventional metrology protocols, like Ramsey interferometry, with critical metrology protocols. Second relies on showing how finite temperature can be, in principle, used to increase the quantum Fisher information and the sensitivity of estimating physical parameters.

Time and date： 10:45-, December 20, 2023

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker： Kiya, Haruki（QC）

Title：Isoholonomic problem and composite quantum gate robust against pulse length error

Abstract：One confronts two typical systematic errors in one-qubit control: Pulse Length Error (PLE) and Off Resonance Error (ORE). To prevent the loss of control accuracy due to these systematic errors, a technique called Composite Quantum Gate (CQG) [1] exists, replacing a single operation with a sequence of several operations. In this talk, we will focus on PLE robust CQGs. The geometry of the PLE robust CQG is related to the Aharonov-Anandan phase [3], an extension of the Berry phase [2]. We argued the shortest operation time by minimizing the length of the path drawn on the Bloch sphere while satisfying the robustness condition against PLE [4]. However, the bound in [4] is not tight and needs more discussions: This is the subject of this talk and is related to the isoholonomic problem [5,6].

[1] Levitt, M. H. (1986). *Progress in Nuclear Magnetic Resonance Spectroscopy*, *18*(2), 61-122.

[2] Berry, M. V. (1984). *Proceedings of the Royal Society of London. A. Mathematical and Physical Sciences*, *392*(1802), 45-57.

[3] Aharonov, Y., & Anandan, J. (1987). *Physical Review Letters*, *58*(16), 1593.

[4] Kukita, S., Kiya, H., & Kondo, Y. (2023). *Journal of Physics A: Mathematical and Theoretical*, *56*(48), 485305.

[5] Montgomery, R. (1990).* Communications in Mathematical Physics*, *128*, 565-592.

[6] Tanimura, S., Nakahara, M., & Hayashi, D. (2005). *Journal of mathematical physics*, *46*(2).

Date and time： 2024年1月10日10:45-

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker： Andou, Kyousuke（CMT）

Title：Machine learning analysis of Fully Frustrated XY Model (FFXY) in two-dimensional square lattice

Abstract：The recent remarkable development of artificial neural networks in image recognition, image classification, and natural language processing has influenced many scientific fields, and the search for new discoveries by applying this technology to any problem has begun. In the field of classical statistical physics, machine learning algorithms were introduced to identify symmetry-broken phase [1-3], and in some of these cases neural networks were shown to be able to learn order parameters and other thermodynamic parameters [1,3]. Having been able to apply machine learning techniques to conventional phase transitions, it is natural to ask whether the algorithm can be applied to unconventional phase transitions. As an example of such a system, we focus on the 2D Fully Frustrated XY Model (FFXY) [4]. The FFXY model has an Ising model-like transition and an XY model-like transition.

The purpose of this study is to see if machine learning algorithms trained by the XY and Ising models can detect phase transitions for the two-dimensional square lattice FFXY model. The two neural networks used were trained from the spin configuration of the Ising model and the vortex configuration of the XY model. The FFXY model detects vortices from spin configurations obtained from Monte Carlo simulations, and inputs them to the learning model.

[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] Stephen Teitel, 40 Years of Berezinskii–Kosterlitz–Thouless Theory (World Scientific), pp. 201-235 (2013).

Time and date： 9:00-, January 17, 2024

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker： Miyai, Seiichiro（QMB）

Title：Toward an analysis of correlation propagation in the Bose-Hubbard model with dipole-dipole interactions

Abstract：Rapid technological advances in preparing and manipulating cold atoms have offered unique opportunities for studies of non-equilibrium dynamics of quantum many-body systems. One of the fundamental questions to be addressed is how correlations propagate in these systems. Specifically, such correlation propagation dynamics have been analyzed in experiments with Bose gases in optical lattices [1,2], which can be well described by the Bose-Hubbard model. Moreover, recent experiments using atoms with strong dipole-dipole interactions [3,4] have opened up new possibilities for studying effects of the long-range interactions on correlation propagation dynamics. In this work, we aim to theoretically investigate correlation spreading in the Bose-Hubbard model with dipole-dipole interactions. In preparation for analyzing this model, this presentation will introduce the auxiliary bosonic operators, which are the basic approximations used in the analysis of the 1D Bose-Hubbard model, and present a part of what has been learned about the approximation of unconstrained fermions (UF approximation) [5].

References:

[1] Marc Cheneau, Peter Barmettler, Dario Poletti, et al., nature10748, NATURE vol481,487,(2012).

[2] Y. Takasu, et al., Science Advances 6, eaba9255 (2020).

[3] S. Baier, M. J. Mark, D. Petter, et al., Science 352, 201 (2016).

[4] Lin Su, Alexander Douglas, Michal Szurek, et al., Nature 622, 724 (2023).

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

Date and time： 10:45-, January 19, 2024

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker 1： Watanabe, Genki（QMB）

Title：Only in Japanese

Abstract：Only in Japanese

[1] M. Saffman, J. Phys. B: At. Mol. Opt. Phys. 49, 202001 (2016).

[2] X. Wu et al., Chinese Phys. B 30, 020305 (2021).

[3] H. Levine et al. Phys. Rev. Lett. 123, 170503 (2019).

[4] S. de Léséleuc et al. Phys. Rev. A 97, 053803 (2018).

Speaker 2： Ueda, Kenta（QMB）

Title：Only in Japanese

Abstract：Only in Japanese

Speaker 3： Iteya, Masatoshi（CMT）

Title：Only in Japanese

Abstract：Only in Japanese

Speaker 4： Fuchi, Shougo（CMT）

Title：Only in Japanese

Abstract：Only in Japanese

Time and date： 9:00-, January 24, 2024

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker 1： Kimura, Ren（QMB）

Title：Only in Japanese

Abstract：Only in Japanese

Speaker 2： Takada, Iori（CMT）

Title：Only in Japanese

Abstract：Only in Japanese

Speaker 3： Matsuo, Hirotaka（CMT）

Title：Only in Japanese

Abstract：Only in Japanese

Time and date： 13:15-, February 15, 2024

Room： Simulation and Experiment Room, 3rd Floor, 31 East Bldg. + Webcast via Zoom

Speaker： Cazalilla, Miguel（Donostia International Physics Center）

Title：Quantum Dissipation in One-Dimensional Quantum Many-Particle Systems

Abstract：I will review several old [1,2] and new results concerning one-dimensional (1D) quantum dissipative systems. Beginning with a brief review of some early studies on the effect of dissipation in Tomonaga-Luttinger liquids [1,2] and spin systems [3], I will conclude by describing some recent results [5] that demonstrate some interesting analogies between the superconductor-metal transition in 1D superconducting wires or Josephson junction arrays and the de confinement of bosons in an anisotropic optical lattice [6,7]. Finally, considering dissipation in out of equilibrium systems, I will describe our recent results on the non-equilibrium dynamics induced by two-body losses of strongly interacting bosons in one-dimensional optical lattices.

[1] M. A. Cazalilla, F. Sols, and F. Guinea, Phys. Rev. Lett. 97, 076401 (2006).

[2] E. Malatsetxebarria, Z. Cai, U. Schollwöck, and MAC, Phys. Rev. A 88, 063630 (2013)

[3] A. M. Lobos, M. A. Cazalilla, and P. Chudzinski, Phys.Rev. B 86 035455 (2012).

[4] C.-H. Huang, T. Giamarchi, and MAC, Phys. Rev. Res. 5, 043192 (2023).

[5] M. A. Cazalilla, to be published.

[6] A. F. Ho, M. A. Cazalilla, and T. Giamarchi, Phys. Rev. Lett. 92, 130405 (2004).

[7] M. A. Cazalilla, A. F. Ho, and T. Giamarchi, New J. Phys. 8, 158 (2006).