Time and date： 15:00-, April 15, 2020

Room： Rm. 301, 3rd Floor, 31 Bldg. + Webcast via Zoom

Speaker： Kaneko, Ryui（QMB）

Title： Magnetic Field Induced Competing Phases in Kitaev Magnets

Abstract： The Kitaev model on a honeycomb lattice is one of the rare examples of exactly solvable models in two-dimensional quantum systems. The Kitaev candidate material alpha-RuCl3 has attracted much interest recently because of the putative field-induced spin liquid. Motivated by the experimental findings, we investigate a ground-state phase diagram of the extended Kitaev model, where the Kitaev and off-diagonal spin-exchange interactions compete.

First, we examine the ground states of the classical model by the simulated annealing method to get an insight into the phase diagram. We find various magnetic orders with substantially large unit cells away from the Kitaev limit. Second, based on the findings in the classical model, we study the quantum model by applying the two-dimensional tensor-network method. When the field and the off-diagonal exchange interaction are sufficiently large, we find two intermediate phases, sandwiched by the ferromagnetic and zigzag phases. They become nonmagnetic as the field decreases and spontaneously break the lattice rotational symmetry.

Video & Slides (PW required)

Time and date： 15:00-, April 22, 2020

Room： Rm. 301, 3rd Floor, 31 Bldg. + Webcast via Zoom

Speaker： Okane, Hideaki（QC）

Title： Quantum remote sensing under the effect of dephasing

Abstract： Quantum remote sensing (QRS), which is proposed in [1], enables us to perform remotely a quantum sensing with a security about the measurement results. In this seminar, we introduce the protocol of the QRS and explain a state preparation error of Bell pair, which is considered in [1]. Next, we discuss our recent research on the QRS, which consider the effect of dephasing as well as the imperfect state preparation. We also show that, as we increase the repetition number, the effect of dephasing become more relevant to the performance of the quantum sensing than the state preparation error.

[1] Y. Takeuchi, Y. Matsuzaki, K. Miyanishi, T. Sugiyama and W. J. Munro; Phys. Rev. A 99, 022325 (2019)

Video & Slides (PW required)

Time and date： 15:00-, May 13, 2020

Room： Webcast via Zoom

Speaker： Kokubo, Haruya（CMT）

Title： Nonlinear dynamics of an interface in phase-separated two-components Bose condensates with counterflow

Abstract： Kelvin-Helmholtz instability (KHI) is an instability that occurs at the insterface of phase-separated fluids with relative velocities. The KHI in Bose-Einstein condensates (BECs) is similar to classical one, since it can be considered as a dynamical instability which occurs without dissipation mechanism of the system[1]. Also,it has been known that two-component superfluids with counterflow cause countersuperflow instability (CSI)when they are in a mixed state[2]. A width of an interface in phase-separated two-component BECs can be controlled by varying the interaction strengths between the atoms. It has been shown that the instability changes like crossover between the KHI and the CSI by changing the interface width[3].

In this talk, I will show how the dynamics of the interface is changed by varying the heterogeneous interaction strength as a parameter characterizing instability for a phase-separated two-component BEC with counterflow without an external potential.

[1] H. Takeuchi, N. Suzuki, K. Kasamatsu, H. Saito, and M. Tsubota, Phys. Rev. B 81, 094517 (2010).

[2] S. Ishino, M.Tsubota, and H. Takeuchi, Phys. Rev. A 83, 063602 (2011).

[3] N. Suzuki, H. Takeuchi, K. Kasamatsu,M. Tsubota, and H. Saito, Phys. Rev. A 82,063604(2010).

Video & Slides (PW required)

Time and date： 15:00-, May 20, 2020

Room： Rm. 301, 3rd Floor, 31 Bldg. + Webcast via Zoom

Speaker： Goto, Shimpei（QMB）

Title： Measurement-induced transitions in ultracold atoms and their detection through the ergodicity breaking

Abstract： Unitary evolution under the Schrödinger equation increases the entanglement of a quantum state. The entanglement of a quantum state is the resource for quantum computations and gives an alternative foundation of statistical mechanics. On the other hand, measurements, nonunitary effects coming from interactions between the environment, collapse a state and decrease the entanglement of the state. Very recently, theoretical studies on quantum circuit models have shown that frequent measurements change the scaling law of the entanglement entropy [1, 2, 3], which quantifies the entanglement. When the rate of measurements is low, the scaling is the volume law, i.e., a state is strongly entangled. If the rate exceeds a critical value, the scaling becomes the area law, i.e., a state is low entangled. Revealing the nature of this measurement-induced transition (MIT) is essential for understanding the effects of measurements on the entanglement and leads to the protection of the entanglement from measurements.

In this talk, I discuss how to realize and detect the MIT in ultracold atoms [4]. Because of the long coherent time and controllability of the isolation, ultracold atoms seem ideal platforms for studying the MIT. Based on quasi-exact numerical simulations with matrix product states, we find that the Bose-Hubbard model with controllable dissipations, which is experimentally realized [5], shows two MITs: a transition from the volume law to the area one at small dissipation and that from the area law to the volume one at very strong dissipation. We also show that MITs can be detected through the ergodicity breaking deduced from the dynamics. Since the ergodicity breaking is the direct consequence of the area-law scaling, the detection scheme we propose can be applied to any particle systems.

References

[1] Y. Li, X. Chen, and M. P. A. Fisher, Phys. Rev. B 98, 205136 (2018).

[2] A. Chan et al., Phys. Rev. B 99, 224307 (2019).

[3] B. Skinner, J. Ruhman, and N. Adam, Phys. Rev. X 9, 031009 (2019).

[4] S. Goto and I. Danshita, arXiv:2001.03400v2.

[5] T. Tomita et al., Sci. Adv. 3, e1701513 (2017).

Video&Slides (PW required)

Time and date： 15:00-, May 27, 2020

Room： Webcast via Zoom

Speaker： Ozaki, Yusuke（QMB）

Title： Semiclassical analysis of Bose gases in kagome lattices with frustration

Abstract： Frustration in physics signifies extensive degeneracy near the ground state of a many-body system. It has received considerable interest because it is an essential component for understanding the various emergent phenomena in many-body systems [1, 2]. For instance, particles on a triangular or kagome lattice may have frustration due to its geometric structure. Specifically, in the case of non-interacting particles on a kagome lattice, a flat band is formed, which is a clear feature of the extensive degeneracy. In this talk, we consider spinless bosons with on-site interaction on the kagome lattice within in the presence of weak quantum fluctuations. Since the single particle energies are degenerate in the flat band, the interaction plays a dominant role on the kagome lattice. In addition, the previous work [3], calculating zero-point energies (ZPEs) due to Bogoliubov phonons for several degenerate states, suggests that the ZPEs resolve the degeneracy and that there exist three different phases depending on the temperature. We discuss a way to numerically examine this proposal by using the truncated Wigner approximation.

[1] H. Diep, *Frustrated Spin Systems* (World Scientific, Singapore, 2004).

[2] R. Moessner and A.P. Ramirez, Physics Today 59, 24 (2006).

[3] Y.-Z. You, Z. Chen, X.-Q. Sun, and H. Zhai, Physical Review Letters 109, 265302 (2012).

Video&Slides (PW required)

Time and date： 15:00-, June 3, 2020

Room： Rm. 301, 3rd Floor, 31 Bldg. + Webcast via Zoom

Speaker： Kukita, Shingo（QC）

Title： “Acrobatic maneuver” in error-robust quantum control of qubit

Abstract： A qubit is a fundamental unit of quantum information. A quantum state of the qubit can be represented as a point in a unit sphere called the Bloch sphere while its dynamics are expressed by trajectories in this sphere. Control of a quantum state of the qubit is performed by unitary operations when we do not consider effects of dissipation. These unitary operations should be robust against systematic errors in control parameters for reliable quantum computing. Composite pulse is a technique to construct unitary operations robust against such systematic errors. The composite pulse basically requires that a quantum state goes through very complicated trajectories in the Bloch sphere. In actual experiments of qubit control, however, there can exist regions that should not be passed: in NMR experiments, for example, when a state of the qubit pass by the equator in the Bloch sphere, the state is disturbed by so-called Radiation Damping. Thus, it is unpreferable that a state pass across the equator in the case of NMR experiments. A question now arises: do there exist composite pulses whose trajectories avoid the equator or such unpreferable regions? In this talk, I shall briefly review some basics of the technique of the composite pulse and investigate the possibility of composite pulses whose trajectories avoid unpreferable regions in the 1-qubit case.

Video&Slides (PW required)

Time and date： 15:00-, June 10, 2020

Room： Webcast via Zoom

Speaker： Machida, Yoshihiro（CMT）

Title： Bose-Hubbard Droplet of two-component Bose Atomic Gases in an Optical Lattice

Abstract： Self-bound quantum droplets are newly discovered phase in the context of ultracold atoms. By using an attractive bosonic mixture, spherical droplets form due to the balance of competing attractive and repulsive forces, provided by first-order correction by quantum fluctuations[1]. It is known that a first-order transition exists at the ground state phase diagram giving a superfluid and a Mott insulator transition in a Bose-Bose mixture system[2]. We aim to realize a spherical droplet in the Bose-Hubbard model, taking advantage of a jump of the condensate density seen in the first order transition points. In this talk, I show the numerical results and future outlook.

Reference

[1] G. Semeghini et al., Phys. Rev. Lett. 120, 235301 (2018)

[2] Y. Kato et al., Phys. Rev. Lett. 112, 055301 (2014)

Video&Slides (PW required)

Time and date： 15:00-, June 17, 2020

Room： Webcast via Zoom

Speaker：Masataka Matsumoto (KEK)

Title： Dynamics of entanglement entropy and disorder in gauge/gravity duality

Abstract： The isolated quantum system exhibits various states in the presence of interactions and disorder. One interesting class is called the many-body localization (MBL) phase, in which the system fails to thermalize. Though the MBL phase is well studied both experimentally and theoretically, it is still controversial whether there exists a phase transition between the MBL phase and the thermalizing phase or not and what parameters characterize the thermalization process.

There is a possibility that the entanglement entropy (EE) can be a universal quantity which distinguishes these two phases and characterizes the thermalization process.

In this seminar, I will talk about a dynamical behavior of the holographic EE in the thermalizing phase and discuss the possibility of realizing the MBL phase in holography.

Video&Slides (PW required)

Time and date： 15:00-, June 24, 2020

Room： Webcast via Zoom

Speaker： Kumoda, Kou（CMT）

Title： CP^1+U(1) lattice gauge model in SU(2) rep.

Abstract： One of quantum models of a brain has been known as the quantum brain dynamics proposed by Takahashi and Umezawa[1] and later extended by Jibu and Yasue [2]. The theory can be analyzed by the four-dimensional CP^1+ U (1) lattice gauge model, which is a quantum field model with a local gauge invariance. The model represents a lattice system with an ensemble of qubits at each site and a electromagnetic field mediates the interaction between qubits.

The phase structure of the CP^1+ U(1) model has been investigated numerically by using Monte Carlo simulation (MC) to study memory mechanism . The results show that this model consists of three phases, the Higgs phase, the Coulomb phase, and the Confinement phase. Each phase is distinguished from the presence or absence of learning and recall ability. In addition, the time development has been studied by the Metropolis method to see the learning and recalling efficiency. As a result, the Higgs phase is possible to learn and recall, and it works correctly as a memory mechanism.

In this talk, we describe the CP^1+U(1) lattice gauge model as a strongly correlated electron system. And, we confirm the real-time dynamics in the one-dimensional CP^1 + U (1) lattice gauge model and introduce the TWA in the representation of SU(2).

[1] C.Stuart, Y.Takahashi and H.Umezawa, J. Theor. Biol.71,pp.605,1978; Found. Phys.9,pp.301,1979. See also G.Vitiello, Int. J. Mod. Phys. B9,pp.973,(1995).

Video&Slides (PW required)

Time and date： 15:00-, July 1, 2020

Room： Rm. 301, 3rd Floor, 31 Bldg. + Webcast via Zoom

Speaker： Danshita, Ippei（QMB）

Title： Spatio-temporal evolution of correlations in the Bose-Hubbard model after a quantum quench

Abstract： A quantum quench means an abrupt change of parameters in the Hamiltonian governing a quantum system. In advanced quantum platforms, such as ultracold gases, trapped ions, Rydberg atoms in optical tweezer arrays, and superconducting circuits, quantum quenches can be easily implemented so that they serve as a standard tool for probing interesting nonequilibrium physics of the quantum many-body systems. Examples that quantum quenches can give rise to include thermalization of isolated quantum systems, propagation of quantum information, and many-body localization. In this work, we analyze nonequilibrium dynamics of the Bose-Hubbard model after a sudden quench by means of a quantum simulator built with ultracold gases in optical lattices [1]. We discuss dynamical spreading of a non-local correlation function in connection with the Lieb-Robinson(-like) bound, which limits the propagation speed of quantum information. We also utilize the outputs from the quantum simulator as a reference for examining quantitative performance of an approximate numerical method, namely the truncated Wigner approximation.

[1] Y. Takasu, T. Yagami, H. Asaka, Y. Fukushima, K. Nagao, S. Goto, I. Danshita, and Y. Takahashi, arXiv:2002.12025v2 [cond-mat.quant-gas].

Video&Slides (PW required)

Time and date： 15:00-, July 8, 2020

Room： Rm. 301, 3rd Floor, 31 Bldg. + Webcast via Zoom

Speaker： Kasamatsu, Kenichi（CMT）

Title： Non-Gibbs states in the Bose-Hubbard system

Abstract： I will talk about the statistical mechanics of discrete nonlinear Schrodinger equation [1]. The analysis shows that the microcanonical dynamics at equilibrium is characterized by two parameters, the particle density and the energy density. There are two regimes in which the usual grand-canonical formalism becomes applicable to one regime, but not to the other; the latter is called the non-Gibbs phase. I will show some characteristics of the non-Gibbs state and that this argument holds for the quantum Bose-Hubbard model (BHM) [2]. Finally, I will show my current project which extends the above study to the BHM with long-range interaction.

[1] K.O.Rasmussen, et al., Phys. Rev. Lett. 84, 3740 (2000).

[2] A.Y.Cherny, et al., Phys. Rev. A 99, 023603 (2019).

Video&Slides (PW required)

Time and date： 15:00-, July 15, 2020

Room： Rm. 301, 3rd Floor, 31 Bldg. + Webcast via Zoom

Speaker： Kondo, Yasushi（QC）

Title： Quantum measurements simulated in NMR

Abstract： Measurements in quantum and classical mechanics are quite different. They can be performed without disturbing a system to be detected in classical mechanics, while it is not the case in quantum mechanics. In order to illustrate this difference, we revisit the quantum teleportation and quantum Zeno experiments [1, 2] in terms of information flow. These experiments were done with a standard high precision NMR spectrometer at Kindai University.

[1] Y. Kondo; JPSJ 76, 104004 (2007).

[2] Y. Kondo, Y. Matsuzaki, K. Matsushima, and J. G. Filgueiras; New J. Phys. 18, 013033 (2016).

Time and date： 16:45-, October 14, 2020

Room： Rm. 301, 3rd Floor, 31 Bldg. + Webcast via Zoom

Speaker： Kukita, Shingo（QC）

Title： Controllable non-Markovianity in phase relaxation

Abstract： Recently, remarkable progress in quantum technology has been witnessed. In view of this, it is important to investigate an open quantum system as a model of quantum devices. Quantum devices often require extreme conditions such as very low temperature for the devices to operate. Dynamics can be non-Markovian in such a situation in contrast with Markovian dynamics in high temperature regime. This observation necessitates us to investigate a non-Markovian open quantum system, both theoretically and experimentally. In this talk, I will report two important results: 1) Exact solution of a simple but non-trivial theoretical model and 2) demonstration of this model by NMR experiments, where non-Markovianity is continuously controllable [1]. We observe qualitative agreement between theory and experiment.

[1] S. Kukita, M. Nakahara, and Y. Kondo, arXiv:2007.11382 [quant-ph].

Time and date： 16:45-, October 21, 2020

Room： Rm. 301, 3rd Floor, 31 Bldg. + Webcast via Zoom

Speaker： Okane, Hideaki (QC)

Title： Spin cooling with a superconducting flux qubit

Abstract： A superconducting flux qubit is a promising sensor with high sensitivity to magnetic fields generated by an electron spin ensemble [1]. To improve the sensitivity, it is desirable for the polarization rate of the electron spin ensemble to be increased. When implementing the sensing protocol with the superconducting flux qubit, the external magnetic fields are typically weak, and the Zeeman energy of electron spins is smaller than the thermal energy. So, the electron spins are considered as completely mixed state. In this talk, we show that the polarization rate can be increased through the interaction between the superconducting flux qubit and the electrons, which we call spin cooling. Specifically, by using spin lock technique for the superconducting flux qubit, the flip-flop interaction between the superconducting flux qubit and the electrons can be induced to cause the energy exchange between them. Our theoretical analysis shows that such a spin cooling can be implemented even with the current technology, and this helps to improve the signal to noise ratio of the superconducting flux qubit to detect the electron spins.

[1]R.P.Budoyo, K.Kakuyanagi, H.Toida, Y.Matsuzaki, and S.Saito; Appl. Phys. Lett. 116,

Time and date： 16:45-, October 28, 2020

Room： Webcast via Zoom

Speaker： Marmorini, Giacomo（Aoyama Gakuin Univ.）

Title： Unconventional phase transitions in SU(N) Heisenberg models under external fields

Abstract： Heisenberg models with enlarged SU(N) symmetry were introduced in condensed matter physics mostly as a theoretical tool that provides calculable approximations for solid-state magnetic insulators. However, with the latest progress in cold-atom experiments with alkaline-earth-like atoms, such as 173-Yb, the expectations to observe the novel type of magnetism associated with higher SU(N) symmetry in the near future have become quite high. In addition to the fully symmetric case, one can also study the situation in which the symmetry is partially broken by external fields that couple to one-body operators in the Cartan subalgebra; this represents a generalization of the external magnetic field in standard spin systems.

In this talk we will focus more in detail on the triangular SU(3) model in a magnetic field [1]. We argue that at zero temperature three nontrivial phases appear below the fully saturated state, thanks to the order-by-disorder mechanism, including a quantum-stabilized 2/3 plateau. The same kinds of phases are stabilized also by thermal fluctuations; quite interestingly, the low-field and high-field phases melt via an unconventional Berezinskii-

If time permits, we will touch upon other systems under investigation, namely the square-lattice SU(3) model with one majority (minority) component [2] and the SU(4) model of coupled tetramers with quadratic Zeeman term [3].

[1] D. Yamamoto et al, PRL 125, 057204 (2020)

[2] H. Motegi et al, in preparation.

[3] Y. Miyazaki et al, in preparation.

Time and date： 16:45-, November 4, 2020

Room： Rm. 301, 3rd Floor, 31 Bldg. + Webcast via Zoom

Speaker： Kaneko, Ryui（QMB）

Title： Tensor-network study of quench dynamics of the two-dimensional Bose-Hubbard model

Abstract：Non-equilibrium dynamics of quantum many-body systems has been a subject of intensive investigation. Measuring the real-time dynamics of two-point spatial correlation functions is worthwhile since it helps understand how quantum information propagates. Analog quantum simulators are ideal devices for measuring such correlation functions; however, examining the accuracy of experiments, at least for simpler systems, is desired through the numerically exact calculations by the classical computers.In this talk, motivated by the recent experiment on a cold-atom quantum simulator [1], we focus on the quench dynamics of a Mott insulating state in the Bose-Hubbard model. In the previous study of real-time dynamics, the single-particle correlation functions observed in the experiment agreed well with those calculated by a numerical method based on matrix product states (MPS) in one spatial dimension. Similarly, in three spatial dimensions, the truncated Wigner approximation (TWA) and the quantum simulators gave nearly consistent results. However, there were no reliable numerical methods in two spatial dimensions. Here we use the two-dimensional tensor network method [2,3] based on projected entangled pair states (PEPS) [4,5], which are the extended version of MPS to two spatial dimensions. We discuss how far one can go using the PEPS algorithm by comparing the experimental and numerical data.

References

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

[2] C. Hubig and J. I. Cirac, SciPost Phys. 6, 031 (2019).

[3] P. Czarnik et al., Phys. Rev. B 99, 035115 (2019).

[4] T. Nishino et al., Prog. Theor. Phys. 105, 409 (2001).