Time and date: 15:00-, April 10, 2019

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Goto, Shimpei（QMB）

Title: Kondo Effects with Electrically Neutral Ultracold Atoms

Abstract: Thanks to recent technical developments in controlling atoms with two outermost electrons and their internal states, analog quantum simulators of multiorbital many-body systems are being realized [1,2]. When quantum simulators are established, a natural next step is simulating some characteristic well-understood physics. In multiorbital many-body systems, the exceptionally well-understood physics is the Kondo effect of the Kondo model: the anomalous temperature dependence of electrical resistivity caused by magnetic impurities [3]. However, ultracold atoms are electrically neutral; How can one measure electrical resistivity?

In this talk, I discuss how to experimentally observe the Kondo effects in ultracold atoms based on our numerical simulations of finite temperature dynamics induced by the sudden shift of trap center. According to the results of our numerical simulations, the center-of-mass velocities, which are measurable in typical experiments, show the anomalous temperature dependence instead of electrical resistivity. The simulations also show that this temperature dependence is absent in the ferromagnetic Kondo model and fully spin-polarized system. These facts strongly indicate that the Kondo effects are detectable by the experimentally measurable center-of-mass velocities.

[1] L. Riegger et al., Phys. Rev. Lett. 120, 143601 (2018).

[2] K. Ono et al., Phys. Rev. A 99, 032707 (2019).

[3] J. Kondo, Prog. Theor. Phys. 32, 37 (1964).

Time and date: 15:00-, April 17, 2019

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Le, Binho（QC）

Title: Introduction to weak measurements and weak values

Abstract: In quantum mechanics, measurements of an observable of a quantum system allow for the prediction of the attentive characteristics of that system. The average value of the observable is described by its operator and the quantum wave function that represents the system state, and is well known as “expectation value.” If the system, however, described by a two-state vector, i.e., an initial state and a final state, such that the system’s initial state is preselected and its final state is postselected, then the corresponding average value is known as “weak value,” which was first named by Aharonov, Albert, and Vaidman in their groundbreaking study in 1988. Weak values mainly go along with weak measurements, which usually require an infinitely small interaction strength.

In this talk, I will introduce the basic concept of weak measurements and weak values. Together with this, I also will introduce some applications of weak values to interpret some quantum paradoxes including the EPR paradox, the Hardy paradox, the Cheshire cat paradox, and the violation of the pigeonhole principle.

Time and date: 15:00-, Apr. 24, 2019

Room: Rm. 301, 3rd Floor, 31 East Bldg.

Speaker: Ozaki, Yusuke（QMB）

Title: Semi-classical dynamics of a dark soliton in a one-dimensional Bose gas in an optical lattice

Abstract: Since a dark soliton in a one-dimensional Bose gas in the superfluid phase is one of main low-energy excitations, its properties have been actively studied both in theory and in experiments [1]. Previous works have also investigated the effects of quantum fluctuations on dark solitons by means of various theoretical methods, such as the Bogoliubov theory, the Bethe ansatz, and matrix product states, because the strength of quantum fluctuations is controllable in this system by tuning the density or the interparticle interation.

The properties of dark solitons in the classical regime qualitatively change when the Bose gas is confined in an optical lattice. Specifically, while a dark soliton is dynamically stable in continuum, it can be dynamically unstable under a lattice. Additionally, there is a remarkable difference in the dynamical stability depending on whether the position of the dark soliton phase kink is on a lattice point or on a lattice junction [2]. As for the effects of quantum fluctuations, the previous analyses using matrix product states have shown that although the dark soliton becomes unstable due to strong quantum fluctuations, the instability does not depend on the position of the phase kink in contrast to the classical regime. This suggests that the picture of the stability of dark solitons changes between these two regimes, namely the classical regime and the regime of strong quantum fluctuations.

In this study, we use the truncated Wigner approximation to analyze numerically the dynamics of dark solitons in the optical lattice in the semi-classical regime. By changing gradually the strength of quantum fluctuations from the classical regime, we reveal that even weak quantum fluctuations significantly affect the stability of dark solitons. From this result, we propose that one can distinguish whether the origin of the dark-soliton instability is classical or quantum by observing the difference in dynamics between the two types of solitons.

References:

[1] D. J. Frantzeskakis, J. Phys. A: Math. Theor. 43 213001 (2010).

[2] R. V. Mishmash, I. Danshita, Charles W. Clark, and L. D. Carr, Phys. Rev. A 80, 053612 (2009).

Time and date: 13:30-, May 16, 2019

Room: Rm. 301, 3rd Floor, 31 Building

Speaker: Yanagi, Itaru（HITACHI）

Title: Development of silicon nitride nanopore sensors for direct DNA sequencing

Abstract: Long-read DNA sequencing with high throughput at low cost is strongly anticipated especially for the realization of personalized medicine in the future. To meet these requirements, DNA sequencing with nanopores has been attracting a lot of attention. Nanopore technology can be categorized broadly into two types according to the constituent materials of the nanopore. One is “biological”, i.e., nanopores that are formed with biological molecules. The other is “solid-state”, i.e., nanopores that are formed with semiconductor-related inorganic materials (“solid-state nanopores”). Solid-state nanopores, which are the focus of our study, have advantages in terms of robustness and possible large-scale integration. However, many issues must be resolved to realize DNA sequencing with solid-state nanopores. In particular, from the standpoint of device fabrication and DNA manipulation, we believe that there are three key issues to be resolved: (1) stable fabrication of a small nanopore with a diameter on the same order as that of DNA, (2) fabricating an ultrathin membrane, and (3) controlling the speed of DNA passing through the nanopore. In this seminar, these three issues will be addressed and discussed.

(*) This seminar is organized formally as an interdisciplinary seminar at Faculty of Science and Engineering, Kindai University.

Time and date: 15:00-, May 22, 2019

Room: Simulation Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Kumoda, Kou（CMT）

Title: Quantum brain dynamics: Real time evolution of CP_{1}+U(1) lattice gauge model

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, I will explain briefly the essence of the quantum brain dynamics and show the results of real-time evolution of the CP_{1} + U (1) lattice gauge model in a one-dimensional lattice by the semiclassical Gross-Pitaevskii (GP) equation.

[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).

Time and date: 15:00-, May 29, 2019

Room: Simulation Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Machida, Yoshihiro（CMT）

Title: Quench dynamics of a superfluid – Mott insulator of ultracold atoms in a two-dimensional optical lattice

Abstract: Research on ultracold atomic gases has been widely spread according to the success of the Bose-Einstein condensation with a vapor of rubidium-87 atoms in 1995 [1]. In particular, experimental techniques called “optical lattice” that can be created by facing laser beams have been attracting great attentions to us in recent years. The interest has been pioneered from the observation of a superfluid and a Mott insulator quantum phase transition in 2002[2].

In this study I focus a quench dynamics from the Mott insulator to the superfluid quantum phase transition by using ultracold atoms trapped in an optical lattice. For quenching from the Mott insulator to the superfluid, I increased the tunneling term linearly with respect to time. In recent years, Kibble-Zurek mechanism (KZM) in homogeneous systems has been applied to the quench dynamics of the Bose-Hubbard model, where the vortex density follows a power law of the quench time . Supposing realistic experiments, I introduced a harmonic trap potential and investigated the inhomogeneous Kibble-Zurek mechanism (IKZM) [3]. I used a Gutzwiller approximation to analyze numerically the dynamics from the Mott insulator to the superfluid quantum phase transition. I will explain the results based on the IKZM.

References:

[1] M.H. Anderson, J.R. Ensher, M.R. Matthews, C.E. Wieman, E.A. Cornell *Science* **269**(5221), 198-201.

[2] M. Greiner, O. Mandel, T. Esslinger, T.W. Hansch, and I. Bloch, *Nature ***415**,39-44(2002)

[3] A del Campo et al. 2011 *New J. Phys. ***13 **083022

Time and date: 15:00-, Jun. 5, 2019

Room: Simulation Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Kasamatsu, Kenichi（CMT）

Title: Disorderless quasi-localization of polar gases in one-dimensional lattices

Abstract: We study non-equilibrium dynamics of one-dimensional polar gases in deep optical lattices, which present a severely constrained dynamics due to the interplay between dipolar interactions, energy conservation, and finite bandwidth [1]. The appearance of dynamically-bound nearest-neighbor dimers enhances the role of the r^{-3} dipolar tail, resulting, in the absence of external disorder, in quasi-localization via dimer clustering for very low densities and moderate dipole strengths. Furthermore, even weak dipoles allow for the formation of self-bound superfluid lattice droplets with a finite doping of mobile, but confined, holons. Our results, which can be extrapolated to other power-law interactions, are directly relevant for current and future lattice experiments with magnetic atoms and polar molecules.

[1] W. Li, A. Dhar, X. Deng, K. Kasamatsu, L. Barbiero, L. Santos, arXiv:1901.09762

Time and Date：13:15-, Jun. 20, 2019

Room： Rm. 401, 4th Floor, 31 Building

Speaker： Matsuzaki, Yuichiro（National Institute of Advanced Industrial Science and Technology）

Title：Quantum IoT Architecture

Abstract：IoT is a concept to connect sensors and computers with an internet so that remote control and information exchange can be performed. IoT has a potential to be used in many places including a hospital, farms, factories, and home. However, since IoT corrects data from our daily life, it is very important to construct a secure system where information should not be leaked with the third party. The traditional cryptography guarantees that it takes long time for the current computers to break, but future computers (such as a quantum computer) could decrypt the code. Here, we introduce a theoretical concept of quantum IoT where the information is not leaked with the third party as long as quantum mechanics is correct. We apply the concept of quantum cryptography to the IoT, and the hardware such as sensors become immune against cyber attack. We use an entanglement between the client (who needs the information) and sensors, and the quantum correlation guarantees that the third party cannot have an access with the information. If there is a time, we would also introduce our experimental progress of quantum IoT.

※ This seminar is organized formally as an interdisciplinary seminar at Faculty of Science and Engineering, Kindai University.

Time and Date： 15:00-, Jun. 26, 2019

Room： Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker： Kondo, Yasushi（QC）

Title：Composite Quantum Gates with Aharanov–Anandan phases

Abstract：Unitary operations acting on a quantum system must be robust against systematic errors in control parameters for reliable quantum computing. Composite pulse technique in nuclear magnetic resonance realizes such a robust operation by employing a sequence of possibly poor-quality pulses. We show that composite pulses that compensate for a pulse length error in a one-qubit system have a vanishing dynamical phase and thereby can be seen as geometric quantum gates with Aharanov–Anandan phases.

Time and Date： 13:15-, Jul. 4, 2019

Room： Rm. 301, 3rd Floor, 31 East Bldg.

Speaker： Masuda, Shumpei (Tokyo Medical and Dental University)

Title：Theoretical study of fast and accurate controls of quantum systems based on shortcuts to adiabaticity

Abstract: In this talk we show a spin-selective coherent electron transfer in a silicon-quantum-dot array. We discuss the engineering of high-fidelity fast two-qubit gate operations for information processing using STIRAP and shortcuts to adiabaticity. In addition, we report the adiabatic and nonadiabatic creation of single microwave photon in a circuit QED system, which paves. We introduce two different kinds of on-chip microwave photon sources with high efficiency.

※ This seminar is organized formally as an interdisciplinary seminar at Faculty of Science and Engineering, Kindai University.

Time and Date： 16:45-, Jul. 17, 2019

Room： Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker： Danshita, Ippei (QMB)

Title：How to use optical-lattice quantum simulators

Abstract: Analog quantum simulators built with ultracold gases in optical lattices are a useful and unique tool for studying quantum many-body physics. However, the usage of the quantum simulators is rather nontrivial in comparison with, e.g., that of classical computers. In this talk, on the basis of my experiences as their heavy user, I will try to describe how theorists can use optical-lattice quantum simulators. I also discuss a future plan for improving user friendliness of the quantum simulators.

Time and Date： 15:00-, Jul. 24, 2019

Room： Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker： Ihara, Kousuke（CMT）

Title：Transverse instability and disintegration of domain wall of relative phase in coherently coupled two-component Bose-Einstein condensates

Abstract: In a typical two-component mixture of Bose-Einstein condensates with coherent Rabi coupling, the soliton can exist as “domain wall of the relative phase”, which is obtained as a solution of the sine-Gordon equation.[1] We study transverse instability and disintegration dynamics of a domain-wall of a relative phase.We obtain analytically the stationary solution of the domain wall and study numerically the energetic and dynamical stability. In the unstable region, the domain wall is always dynamically unstable for the transverse modulation.

[1]Son and Stephanov, Phys. Rev. A 65, 063621 (2002)

Time and Date： 15:00-, Jul. 31, 2019

Room： Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker： Nakahara, Mikio（Shanghai University）

Title：Quantum links in Bose-Einstein condensates created by counterdiabatic control

Abstract:Creation of link structures in the polar phase of spin-1 Bose-Einstein condensates using the counterdiabatic protocol is theoretically studied. We provide an analytic solution to the evolution of the external magnetic field that is used to imprint the links. As confirmed by our simulations using the full 3-d spin-1 Gross-Pitaevskii equation, our method allows for the precise control of the Hopf charge as well as the creation time of the links. The links with Hopf charge exceeding unity display multiply nested Hopf links.

Reference: T. Ollikainen, S. Masuda, M. Möttönen and M. Nakahara, Phys. Rev. A 96, 063609 (2017).

Time and date: 10:45-, October 11, 2019

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Uno, Takashi (QMB); Kokubo, Haruya (CMT); Yokoi, Mari (QMB)

Title: Only in Japanese

Abstract: Only in Japanese

Time and date: 16:45-, October 23, 2019

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Le, Binho (QC)

Title: Novel development of quantum post-selection measurement for applications in quantum sensing

Abstract: In quantum mechanics, measurements of an observable enable us to predict the attentive characteristics of a quantum system. The average value of an observable is described by its operator and the quantum wave function representing the system state. If the system is characterized by a two-state vector, including the preselected initial state and the post-selected final state, the corresponding average value is known as “weak value.” More informative than the usual average value, weak value is used in a wide range from fundamental science to modern technologies.

In this talk, I will first give an overview of post-selection measurements from which weak value is obtained. Then I will present our recent development of post-selection measurements for quantum sensing. In particular, I will show that the precision in quantum sensing can be enhanced by using post-selection measurements. For multi-parameter, it allows for achieving the ultimate prediction of all parameters simultaneously. I will also present some progress in our current works on quantum sensing. Finally, I will briefly discuss.

Refs.

[1] LB Ho and Y Kondo, Phys. Lett. A 383 153 (2019).

[2] LB Ho and Y Kondo, arXiv: 1811.08046

Time and date: 16:45-, October 30, 2019

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Goto, Shimpei (QMB)

Title: Measurement-induced transitions in the Bose-Hubbard models with controllable dissipations

Abstract: Unitary evolution in quantum mechanics thermalizes a quantum state: the expectation values of local operators given by a pure state become close to the thermal expectation values given by the thermal density matrix characterized by the energy of the pure state. The thermalization also ensures that long-time evolved states are strongly entangled and that their entanglement entropies obey the volume-law scaling likewise the thermal entropy. On the other hand, non-unitary operations like measurements may destroy the entanglement of quantum states. Recent studies of quantum circuit models have theoretically shown that the competition between unitary dynamics and measurements introduces the transition of the scaling law which the entanglement entropy obeys [1, 2, 3]. However, it is almost impossible to experimentally observe the new transition in quantum circuits at the present: several tens of entangled quantum bits are required for the observation.

In this talk, I discuss the possibility of experimental observations of the new transition in ultracold gases. In ultracold gases, we can prepare entangled several tens or hundreds of atoms and the Bose-Hubbard models with controllable dissipations have been realized [4]. From numerical simulations based on matrix product states, we find that there also exists the transition of the scaling law in the dissipative Bose-Hubbard models. We also observe the reentrant behavior of the transition because of the quantum Zeno effects. Furthermore, the quantum Zeno effects also significantly modify the experimentally observable momentum distribution. These findings will lead to the experimental observation of the new transitions.

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

Time and date: 16:45-, November 6, 2019

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Machida, Yoshihiro (CMT)

Title: The effect of a Mott lobe structure on a superfluid-Mott insulator transition

Abstract: In this study I focus a quench dynamics from a Mott insulator (MI) to a superfluid (SF) quantum phase transition by using ultracold atoms trapped in an optical lattice. The inhomogeneous systems can be realized by introducing harmonic trap potential. In this system, I simulate the non-equilibrium dynamics of the MI-SF transition.

I find that the presence of the Mott lobe in the phase diagram of the Bose-Hubbard model provides new features of the inhomogeneous Kibble-Zurek mechanism (IKZM) [1], where the “local” quench time has a nontrivial dependence of the radial coordinate. I discuss the location of quantum vortices created during quench from a MI to a SF. As a future prospect, I briefly describe the phase diagram of the SF-MI in the two-component Bose system [2].

References

[1]A del Campo, A Retzker, and M B Plenio, 2011 *New J. Phys. ***13 **083022 (2011)

[2] Y. Kato, D. Yamamoto, and I. Danshita, Phys. Rev. Lett. **112**, 055301 (2014)

Time and date: 15:00-, November 14, 2019

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

QC group’s mini-workshop

Time and date: 16:45-, November 20, 2019

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Kumoda, Kou (CMT)

Title: CP^1+U(1) lattice gauge model

Abstract: Last time, we analyzed the dynamics of magnetization in the one-dimensional lattice gauge model of CP^1+U(1) by classical approximation. We study what phase structures can be obtained by real-time dynamics involving extending to a three-dimensional lattice model and quantum fluctuations.

In this talk, we briefly describe the CP^1+U(1) lattice gauge model as a strongly-correlated electron system and discuss the harmonic oscillator as an introduction to the TWA (truncated Wigner approximation) used to incorporate quantum fluctuations.

Time and date: 13:15-, November 28, 2019

Room: Rm. 301, 3rd Floor, 31 Building

Speaker: Yamamoto, Daisuke (Aoyama Gakuin Univ.)

Title: Frustrated quantum magnetism of antiferromagnetic insulators and their quantum simulations with cold atoms

Abstract: “Magnetism” plays an integral role for daily life in modern society, for example, as HDD storages and magnetic sensors. The microscopic origin of the magnetic properties of materials is a quantum-mechanical degree of freedom, called “spin.” Nevertheless, the permanent magnets we usually see, such as Iron and Cobalt, do not exhibit a quantum-mechanical behavior, and thus can be simply described in terms of classical-vector spins. Of current interest in the research field of magnetism is to explore an exotic quantum magnetism and its quantum control. In particular, antiferromagnetic materials, in which the neighboring spins in the crystal tend to align in anti-parallel each other, can possess a non-classical property due to the so-called geometrical frustration. In the seminar, we discuss the quantum magnetism observed in newly-synthesized materials, including Ba_{3}CoSb_{2}O9 and Ba_{2}CoSi_{2}O_{6}Cl_{2}, and review the theoretical analyses with numerical calculations on the experimental observations. Besides, we would see the latest topics in a new direction of the magnetism research with the use of quantum simulators constructed with laser beams and cold atomic gases.

(*) This seminar is organized formally as an interdisciplinary seminar at Faculty of Science and Engineering, Kindai University.

Time and date: 16:45-, December 4, 2019

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Ihara, Kousuke (CMT)

Title: Twist structure of the vortex molecule

Abstract: In Two-component Bose-Einstein condensates with different spin states that are coherently Rabi-coupled, it is known that there exists a domain wall with a characteristic that the relative phase changes by 2π as a metastable structure. [1] Suppose now that there is a coreless vortex in each component, which has a core in one component and the other component fills the core.At this time, the domain wall connects the two vortices to form a structure of vortex molecules.Last time, we considered vortex molecules in two dimensions. This time I would like to extend it to three dimensions.In the three-dimensional vortex molecule, the domain wall connects the two vortex yarns.A twiston structure in which this vortex molecular structure is twisted in the vertical direction is expected in three dimensions.In fact, twiston of vortex molecule have been found in superfluid He3-B phase and two-band superconductors. [2][3] This time, we will present the details and the results of numerical calculations in coherently coupled two-component Bose-Einstein condensates.

[1] Son and Stephanov, Phys. Rev. A 65, 063621 (2002)

[2] Y.Kondo, J.S.Korhonen, M.Krusius, V.V.Dmitriev, Y.M.Mukharsky, E.B.Sonin, and G.E.Volovik, Phys. Rev. Lett. 67, 81 (1991)

[3] Tanaka Y, Crisan A, shivagan D D, Iyo A, Tokiwa K and Watanabe T Japan. J. Appl. Phys. 46 134 (2007)

Time and date: 15:00-, December 12, 2019

Room: Rm. 808, 8th Floor, 31 Building

Speaker: Aritomo, Yoshihiro (Kindai University)

Title: Only in Japanese

Abstract: Only in Japanese

Time and date: 13:15-, December 19, 2019

Room: Rm. 301, 3rd Floor, 31 East Bldg.

Speaker: Masui, Takahiko (Kindai University)

Title: Micro-ARPES measurements of the surface electronic state in Y123/Y124

Abstract: In this talk, I briefly review the electronic properties of cuprate

superconductors, and introduce recent collaborative research, micro-ARPES (angle-resolved photoemission spectroscopy). There exist two types of surface termination on a cleaved surface of cuprate superconductor Y123/Y124. Spatially resolved micro-ARPES measurements distinguish the difference of the electronic state due to the surface termination.

[1] H. Iwasawa et al., Phys. Rev. B 98, 081112(R) (2018).

[2] H. Iwasawa et al., Phys. Rev. B 99, 140510(R) (2019).

Time and date: 16:45-, December 25, 2019

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Ozaki, Yusuke (QMB)

Title: Semiclassical analysis of frustrated Bose gases in optical kagome lattices

Abstract: Frustration in physics means extensive degeneracy near the ground state of a many-body system. It has attracted considerable theoretical interest because it is essential elements to understand an essential element for understanding various emergent phenomena in the many-body system many-body systems [1, 2]. For instance, particles on a triangular or kagome lattice may have frustration resulting from its geometric structure. Specifically, in the case of non-interacting particles on a kagome lattice, the lowest band forms a flat band, which is a clear signature of the frustration, when the hopping of the particles is negative.

In this talk, we consider spinless bosons with on-site interaction on the kagome lattice under sign-inverted hopping within weak quantum fluctuations. Since the single particle energies are degenerate inside the flatband, the interaction plays a dominant role in a kagome lattice. Thus, we focus on a weak-interaction and high-density regime where quantum fluctuations are weak. We review the previous work [3], which revealed that the single particle degeneracy is lifted by the Hartree energy within the mean-field approximation. The work also calculated zero-point energies (ZPEs) of Bogoliubov phonons for several degenerate states in order to suggest that the ZPEs resolve the degeneracy and 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, {\it 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).

Time and date: 16:45-, January 8, 2020

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Uno, Takashi (QMB)

Title: Only in Japanese

Abstract: Only in Japanese

Time and date: 10:45-, January 10, 2020

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Yokoi, Mari (QMB)

Title: Only in Japanese

Abstract: Only in Japanese

Time and date: 10:45-, January 17, 2020

Room: Numerical Experiment Room, 3rd Floor, 31 East Bldg.

Speaker: Kokubo, Haruya (CMT)

Title: Only in Japanese

Abstract: Only in Japanese