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冬学期 / Winter semester (October 2015-January 2016)

火曜日13:00から理学部１号館233号室で行います(普段と場所or時間の異なる場合は赤字で示します)。
Every seminar starts from 13:00, Tuesday @ #233, Faculty of Science Bldg. 1 (except when indicated otherwise).

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Oct. 13 (No Seminar)
Oct. 20 Shunsuke Furukawa / Naoto Tsuji
Oct. 27 Ken Funo / Yuto Ashida
Nov. 3 (National Holiday)
Nov. 10 Sho Higashikawa / Shun Uchino
Nov. 17 Eriko Kaminishi / Zongping Gong
Nov. 24 Kohaku So (Hongbo Zeng) / Yusuke Horinouchi
Dec. 1 Tomohiro Shitara / Yuto Murashita
Dec. 8 (No Seminar)
Dec. 15 (No Seminar)
Dec. 21 Shuhei M. Yoshida / Naoto Kura
Dec. 22 Ryusuke Hamazaki / Ulrich Ebling
Jan. 12 (No Seminar)
Jan. 19 (No Seminar)
Jan. 26 Kenji Okubo / Tomoya Hayata (@ #1220)

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speaker	Shunsuke Furukawa
title	Excitation band topology and edge matter waves in Bose-Einstein condensates in optical lattices
abstract	Topological insulators and superconductors have attracted great attention in recent years for their rich variety of quantized responses and robust edge states originating from nontrivial bulk band topology. Ultracold atomic systems have recently emerged as a new platform for exploring the physics of topological phases, especially owing to ongoing experimental developments for engineering synthetic gauge fields required to produce such phases. While the main focus of the studies of topological phases has been placed on fermionic systems, atomic systems offer unique opportunities to study their bosonic counterparts in a controllable manner. In the noninteracting case, topological properties of Bloch bands do not depend on quantum statistics. It is interesting to ask how the topological properties of Bloch bands are carried over to Bogoliubov excitation bands, which are the elementary excitations of weakly interacting Bose-Einstein condensates (BEC) in optical lattices. Here we study such topological properties of Bogoliubov excitation bands in BECs, using a Bose- Hubbard extension of the Haldane model on a honeycomb lattice. We show that the topological properties of Bloch bands for the noninteracting case are smoothly carried over to Bogoliubov excitation bands for the interacting case, and that the parameter ranges displaying topological bands enlarge with increasing the Hubbard interaction . In the presence of sharp boundaries, chiral edge modes appear in the gap between topological excitation bands. We demonstrate that by coherently transferring a portion of a condensate into an edge mode, a density wave is formed along the edge due to an interference with the background condensate. We propose to use this as a macroscopically enhanced signature of an edge mode. Reference: S. F. and M. Ueda, arXiv: 1506.04556

speaker	Naoto Tsuji
title	Nambu's mysterious relation on Higgs mode in superconductors
abstract	Higgs mode, that I focus on in this talk, is an elementary collective excitation of the amplitude oscillation of the order parameter in superconductors. It is so called by analogy with Higgs particle in high energy physics. The ratio between the 'mass' of the Nambu-Goldstone mode, Bogoliubov quasiparticles, and Higgs mode is known to be 0:1:2 in the simplest case. This, and its generalization, intimately relates bosons and fermions that are far apart from each other, as conjectured by Nambu in 1985. I present a survey on it, and give a few results from a point of view of the wave function. The symmetry principle behind Nambu's relation is still in a cloud.

speaker	Ken Funo
title	Work fluctuation-dissipation trade-off in heat engines
abstract	Recent developments of nonequilibrium statistical mechanics allow us to formulate thermodynamic relations for arbitrary nonequilibrium initial and final states [1]. They can be used to quantify thermodynamic costs of information encoding and erasure processes as well as to quantify the extractable work from information heat engines. In those general situations, reducing energy dissipation allows us to increase the efficiency of a given thermodynamic task, and reducing work fluctuation allows us to inject into the system an exact amount of work needed to complete the task, or to extract a deterministic amount of work from the system. Thus, suppressing both work fluctuation and energy dissipation is vital to control nanosystems that work at the level of thermal fluctuations. Previous studies have explored the regime around vanishing work fluctuations by using techniques of quantum information theory, known as the single-shot statistical mechanics [2, 3] and the regime around vanishing energy dissipation by using the second law of thermodynamics [1]. However, as we prove in the present work, these two aims are incompatible. We derive the fundamental trade-off relation between work fluctuation and energy dissipation in heat engines starting from and ending at arbitrary nonequilibrium states and that the lower bound is quantified by the degree of nonequilibriumness of the initial and final states as measured in terms of the information distance [4]. We propose a method to construct explicit protocols that achieve the lower bound of the trade-off relation. An application of the trade-off relation to information heat engines is carried out, including a numerical simulation to test the trade-off relation. [1] M. Esposito and C. Van den Broeck, Euro. Phys. Lett. 95, 40004 (2011). [2] J. Aberg, Nat. Commun. 4, 1925 (2013). [3] M. Horodecki and J. Oppenheim, Nat. Commun. 4, 2059 (2013). [4] K. Funo and M. Ueda, arXiv:1508.04042.

speaker	Yuto Ashida
title	Multi-Particle Quantum Dynamics under Continuous Observation
abstract	Indistinguishability of identical particles and the measurement back-action are two fundamental pillars of quantum mechanics. Yet, a clear understanding of how the interplay between these two postulates affects the quantum many-body dynamics is missing. Regarding the recent developments of in-situ imaging techniques of cold atoms, such a fundamental problem now becomes an experimentally relevant question. In this talk, I will introduce the theory for describing multi-particle dynamics under continuous monitoring, and show that the measurement indistinguishability can protect the system from relative positional decoherence. I will also discuss some novel effects of quantum statistics on the diffusive behavior of atoms trapped in an optical lattice. Reference: Y. A. and M. Ueda, arXiv: 1510.04001

speaker	Sho Higashikawa
title	On the non-abelian nature and topologial interaction of topological excitations
abstract	Topological excitations are ubiquitous in nature, from particle physics and nuclear physics to condensed matter physics. It is known that interaction characterized by topological values exists between topological excitations, such as vortex-vortex interaction and vortex-monopole interaction. Dispite the ubiquitousness and importance of them in nature, little is known about the interaction between them. In particular, only one type of interaction is known for the interaction between vortex and higher dimensional toporolgical excitation for more than 30 years. In this talk, I will talk about general condition for the interaction to be nontrivial. First, I will derive the general formulae for the homotopy group, \pi_i(G/H). Next, by applying these formulae, I will derive the necessary and sufficient condition for a nontrivial interaction between vortices and the necessary condition for a nontrivial interaction between vortex and higher dimensional topological excitation in nature. Finally, I will present a new and exotic vortex-monopole interaction in 3 color density wave phase in SU(3)-symmetric spin model, where 6 types of vortices and 3 types of monopoles appear. In this phase, 3 kinds of monopoles interchange with each other under the interaction with vortices and therefore the vortex-monopole interaction is characterzed by non-abelian group, S_3(permutation group of 3 elements).

speaker	Shun Uchino
title	From 10^(-9) to 10^(32) Kelvin
abstract	In this seminar, I introduce a highlight of my past research, and also show what I am looking at. Emphasis will be on spinor Bose-Einstein condensation, one dimensional systems, and transport in cold atoms.

speaker	Eriko Kaminishi
title	Entanglement pre-thermalization in a one-dimensional Bose gas
abstract	We studied the pre-thermalization in the Lieb-Liniger model and the Tomonaga-Luttinger model. In the Lieb-Liniger model, it is made clear that the pre-thermalization can be understood as a phenomenon due to an influence of the initial entanglement between subsystems. We call it the entanglement pre-thermaliation. On the other hand, in the Tomonaga-Luttinger model, it has been argued that the pre-thermalization occurs due to the presence of many conserved quantities in the literatures. However, in this talk, I will give a new interpretation of the pre-thermalization in the Tomonaga-Luttinger model; I will show you that this pre-thermalization can be also understood in terms of the initial entanglement between the subsystems. Therefore, the pre-thermalization in the Tomonaga-Luttinger model, which was observed in experiment, is also a kind of entanglement pre-thermalization. I will also show you that the spin-charge separation explains the reason why there are two interpretations. In this talk, I will start from explaining the general mechanism of the entanglement pre-thermalization, and then, I will apply it to the Lieb-Liniger and the Tomonaga-Luttinger models.

speaker	Zongping Gong
title	Brownian Szilard Engine
abstract	There are few analytically solvable models in classical stochastic thermodynamics. The only exception for Langevin systems seems to be the Brownian harmonic oscillator in the overdamped regime. It is somehow surprising that an analytical treatment of the thermodynamics of a Brownian particle in a piston (time-dependent rigid wall potential) is still missing. Since such system can be regarded as a standard model of the Szilard engine, it may be named by the Brownian Szilard engine (BSE). The difficulty to solve the BSE model probably results from the singularity of the rigid wall potential, as well as the unconventional work accumulation via discrete collisions. However, it is found that the BSE can be mapped into a boundless and collision-free diffusive system after a special variable transformation, which eliminates the difficulty. A reduced Feynman-Kac equation only in terms of a momentum-like variable is derived under the frequent collision approximation. This equation turns out to be highly similar to the Feynman-Kac equation of an overdamped breathing Brownian harmonic oscillator. Based on the reduced Feynman-Kac equation, the work distribution and the optimal protocol that maximizes the extracted mean work are analytically obtained in the linear response regime.

speaker	Kohaku So (Hongbo Zeng)
title	Phase diagram of ferromagnetic spinor bosons in an optical lattice
abstract	Recently, cold atoms with spin degrees of freedom have attracted considerable interest because of the possibility they offer of modelling quantum magnetism and exploring the interplay between spatial and spin degrees of freedom. While spinor bosons with antiferromagnetic interaction loaded in optical lattices have been widely studied in this context because of their properties such as an even-odd effect in the superfluid to Mott-insulator transition, those with ferromagnetic interaction has not been studied extensively. However, mean-field analysis in the continuum systems suggests that the competition between an external magnetic field and the ferromagnetic interaction could give rise to new and rich phases. We have studied ferromagnetic spinor bosons in an optical lattice under an external magnetic field. Using the decoupling mean-field approximation, we have obtained a rich ground-state phase diagram, in which, in addition to the well-known Mott-insulator and superfluid phases, polar and broken-axisymmetry superfluid phases arise. We also found that the transition between broken-axisymmetry superfluid phase and other phases is a first-order one across some part of the phase boundary, in remarkable contrast to the case without external magnetic fields.

speaker	Yusuke Horinouchi
title	Renormalization-group limit cycle and universal Efimov physics
abstract	In 1970s, Efimov predicted a universal phenomenon of 3-body systems with resonant interaction. The universal phenomenon, today known as the Efimov effect appears in wide range of physical systems such as nucleons, magnons, excitons, and macromolecules such as DNA. One remarkable feature of the Efimov effect is the discrete scale invariance, which means that the energy level forms a geometric series, the scaling factor 22.7 of which does not depend on short-range details of microscopic Hamiltonians. Reflecting the discrete scale invariance, the universality class of the Efimov effect is characterized by a renormalization-group limit cycle, where RG flows form periodic circle rather than a fixed point. In this talk, I will present my recent works which relates some universal properties of the Efimov effect and the renormalization-group limit cycle. As fixed-point analyses have shown universalities in critical phenomena, limit cycle is found to describe the universal physics of quantum few-body systems.

speaker	Tomohiro Shitara
title	How to determine the quantum Fisher metrics from linear response theory through generalized covariance
abstract	The quantum Fisher metrics are the family of monotone metrics on the quantum state space, and give upper bounds on the accuracy of state estimation. The operational meaning of the quantum Fisher metrics beyond the monotonicity has been studied individually, and even how the general quantum Fisher metrics are related to observable quantities has not been known yet. In this talk, we present the method of determining general quantum Fisher metrics. We derive a generalized fluctuation-dissipation theorem that shows the relation between the linear response function and generalized covariances, which are an equivalent description of the quantum Fisher metrics. Based on this theorem, we show that the quantum Fisher metrics can be determined by measuring the linear responses to the harmonically oscillating external field with all frequencies.

speaker	Yuto Murashita
title	Overdamped stochastic thermodynamics with multiple reservoirs
abstract	The overdamped approximation is a powerful tool to quantitatively investigate thermodynamic properties of small systems in an isothermal environment. However, when a system is simultaneously coupled to two or more reservoirs with different temperatures, the overdamped Langevin equation fails to evaluate heat flows. This is because the momentum degrees of freedom relax to a nonequilibrium steady state and therefore contribute to the heat flows even after the relaxation. In this seminar, we construct the overdamped approximation based on the generating function of the heat flows by starting from underdamped stochastic thermodynamics. We illustrate usability of our method in an analytically solvable model.

speaker	Shuhei M. Yoshida
title	Universality and Anisotropy in a Resonantly Interacting p-Wave Fermi gas
abstract	I am basically interested in universal physics that emerges in strongly correlated atomic gases. One important class of universal properties in such systems is a series of equations that bridge macroscopic physics characterized by (e.g.) thermodynamic functions and microscopic behavior characterized by high-momentum or short-range asymptotes of correlation functions. An essential notion here is the contact, a quantity which, together with several universal exponents and numerical constants, determines both long- and short-range behaviors of the system. They are most extensively studied in the unitary Fermi gas, and are extended to low-dimensional systems, lattice systems, and the unitary Bose gas, all of which have excluded anisotropic interaction. In the seminar, I will discuss such universal relations in a spinless Fermi gas with a resonant p-wave interaction. An important distinction between this and previous studies is that the p-wave interaction can be anisotropic. I will introduce a notion of the p-wave contact tensor as the coefficient of the asymptote of the momentum distribution, which reflects the anisotropy of the p-wave interaction. Its diagonal components are shown to be thermodynamic functions, while its off-diagonal components represent a kind of “macroscopic coherence” in the system. As candidate systems in which the off-diagonal components do not vanish, I will discuss an anisotropic p-wave superfluid and non-equilibrium dynamics after the sudden sweep of the Feshbach resonance.

speaker	Naoto Kura
title	A review of, and an insight to, Quantum Machine Learning
abstract	Machine learning has now become the central concern in infomatic science. In this talk, I will introduce the mutual application between quantum machanics and machine learning. Grover's algorithm (1996) demonstrated the power of quantum computation in identifying one record from a huge number of data. This searching method is seen as a special case of pattern recognition, the most fundamental field in machine learning. I will going to review mainly on a paper by Selvedio and Gortler (2004) who compared the power of Quantum pattern recognition to Classical one. They found an asymptotic lower bound for the training data (resource for learning) that any quantum algorithm must take. This means that quantum speedup can only be polynomial (from N to N1/k ), not exponential (from N to log N ). The speed limit of the quantum algorithm is obtained geometrically; one needs to travel a certain distance, but with each training data one can just make a short stride. Grover's algorithm is special in the sense one travels straight the geodesic line. My plan is to consider on other cases in which such line can be traced. -->

speaker	Ryusuke Hamazaki
title	Generalized Gibbs ensemble in a nonintegrable system with an extensive number of local symmetries
abstract	Recent experimental realizations of almost isolated quantum systems have encouraged theorists to study nonequilibrium dynamics that follow unitary time-evolution. One of the important topics is whether systems approach stationary states that are described by the canonical ensemble. In nonintegrable systems that conserve energy alone, the canonical ensemble are expected to be justified by the eigenstate thermalization hypothesis (ETH). In contrast, stationary states in integrable systems are described not by the canonical ensemble, but by the generalized Gibbs ensemble (GGE), due to the existence of many nontrivial conserved quantities. Then, the natural question is how many conserved quantities are required for the GGE to describe the stationary state. In this seminar, we show that the GGE is necessary to describe stationary states even in a nonintegrable system if it has an extensive number of local symmetries. We numerically study a nonintegrable model of hard-core bosons with an extensive number of local Z_2 symmetries that lead to many conservation laws. We show that the expectation values of local observables in the stationary states are described by the GGE instead of the canonical ensemble. We argue that this is because the ETH holds true for each symmetry sector, not for the entire spectrum. Next, we modify the model so that it involves only one global Z_2 symmetry or an L-independent number of local Z_2 symmetries. We show that the canonical ensemble works and that the GGE is not necessary in these models. Reference: R. H., T. N. Ikeda and M. Ueda, arXiv: 1511.08581

speaker	Ulrich Ebling
title	Spinor dynamics of dipolar gases
abstract	I will present some features of ultracold gases with dipole-dipole interactions. This interaction is long-ranged, anisotropic and couples spin and orbital degrees of freedom and leads to interesting effects. First, dipole-dipole interactions do not conserve total magnetization and can lead to a transfer between spin and orbital angular momentum, similar to the Einstein-de Haas effect. Second I will present the effect of Fermi surface deformation which appears in a dipolar Fermi gas because the anisotropic interactions combined with exchange interaction leads to a deformation of the Fermi surface. I will present the theoretical description of the time-evolution for dipolar gases in Hartree-Fock approximation, which leads to a Gross-Pitaevskii-Equation (for dipolar BEC) or a Boltzmann-Vlasov-Equation (for a dipolar Fermi gas) and use numerical results to illustrate Fermi surface deformation and the Eisntein-de Haas effect. Last, I will discuss the two kinds of nematic order (liquid-crystalline order) that can appear in dipolar gases and a possible analog of the Einstein-de Haas-Effect regarding the conversion of spin nematic order into its orbital counterpart.

speaker	Kenji Okubo
title	Isothermal-Isobaric System in Informational Thermodynamics
abstract	As an analogy of the integral fluctuation theorem about isothermal system, formulation of the isothermal-isobaric integral fluctuation theorem has been introduced. This formulation (obviously) enables the discussion of informational thermodynamics to be applied to chemical systems, which is described by Gibbs energy. Especially, in this sentence, catalytic (enzymatic) can be recognised as feedback system (Maxwell’s demon) theoretically. Some real catalytic (enzymatic) reactions leads some examples on this theory.

speaker	Tomoya Hayata
title	Imbalanced Weyl points in three dimensional optical lattice: Chiral magnetic effect in synthetic gauge fields
abstract	I will talk about an ongoing work on quantum simulation of chiral magnetic effect and chiral magnetic wave by using synthetic gauge fields. I study the dynamics of Weyl points in a three dimensional optical lattice by numerically solving the kinetic theory in the presence of a finite Berry curvature. When parallel synthetic electric and magnetic fields are applied, the population of Weyl points can be imbalanced, which generates so called chiral chemical potential. At finite chiral chemical potentials, there are anomalous transport induced by quantum anomaly and accompanying gapless collective excitation, which are, respectively, known as the chiral magnetic effect and chiral magnetic wave. By analyzing the time-evolution of Weyl points at finite synthetic electric and magnetic fields, I will show how they can be observed in a ultracold atom system.

夏学期 / Summer semester (April-July 2015)

木曜日13時から理学部１号館913号室で行います(普段と場所or時間の異なる場合は赤字で示します)。
Every seminar starts from 13:00, Thursday @ #913, Faculty of Science Bldg. 1 (except when indicated otherwise).
<Schedule>
June 2 (Tue) Ulrich Ebling / Tomoya Hayata [@201b, 12:30-]
June 5 (Fri) Prof. Adolfo del Campo [@431, 13:00-]
June 9 (Tue) Prof. Simone De Liberato [@201b, 13:30-]
August 24 (Tue) Prof. Christian Van den Broeck [@913, 10:30-]

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April 16[14:30-] (Thu) Tomohiro Shitara
    "Monotone metrics on matrix spaces"
   D. Petz, Linear algebra and its applications 244 (1996): 81-96.

April 23 (Thu) Shuhei M. Yoshida
    "Boson localization and the superfluid-insulator transition"
   Matthew P. A. Fisher, Peter B. Weichman, G. Grinstein, and Daniel S. Fisher Phys. Rev. B 40, 546

April 30 (Thu) Kohaku So (Hongbo Zeng)
    "Symmetry breaking and finite size effects in quantum many-body systems"
   Koma and Tasaki, J. Stat. Phys. 76 (1994) 745-803:

May 12 (Tue)[@201a,13:30] Ken Funo
    "Path integral for gauge theories with fermions"
   Kazuo Fujikawa, Phys. Rev. D 21, 2848

May 14 (Thu) Yusuke Horinouchi
    "Quasi-Particles and Gauge Invariance in the Theory of Superconductivity"
   Yoichiro Nambu, Phys. Rev. 117, 648

May 21 (Thu) (no seminar)

May 28 (Thu) Yuto Murashita
    "Dynamical Ensembles in Nonequilibrium Statistical Mechanics"
   G. Gallavotti and E. G. D. Cohen, Phys. Rev. Lett. 74, 2694

speaker	Ulrich Ebling
title	Dynamics of Spinor Fermions
abstract	I will present my previous work about the dynamics of harmonically trapped ultracold Fermi gases with large spin. Compared to the spin-1/2 case, large spin fermions must have one of two possible new properties. Either they obey an enlarged SU(N) symmetry, or they feature spin-changing collisions. In my talk, I will address the latter case for the weakly interacting scenario. Because of a different order of magnitude of the scattering lengths for spin-changing and spin-conserving collisions, the system can be in different collision regimes (collisionless, intermediate, hydrodynamic) at the same time. To describe the dynamics of such a system, I use a semi-classical Boltzmann equation with full spin coherence, which treats the single-particle dynamics as an open system coupled to an environment given by all other particles. With this approach, I studied the effect of a strong harmonic confinement [1], which suppresses spatial dephasing and leads to a collective spin dynamics even for fermions, as well as collective excitations (spin waves) [2], large amplitude spin oscillations [3] and the relaxation dynamics of large-spin Fermi gases [4]. The latter projects were done in collaboration with the experimental group of Klaus Sengstock in Hamburg and showed remarkable agreement of theory and experiment. In the last part I will discuss the role of temperature and dimensionality on collision processes in such systems [5]. [1] UE, A. Eckardt and M. Lewenstein, Phys. Rev. A 84, 063607 (2011) [2] J. Heinze et al., Phys. Rev. Lett. 110, 250402 (2013) [3] J. S. Krauser et al., Science 343, 157 (2014) [4] UE et al., Phys. Rev. X 4, 021011 (2014) [5] UE and A. Eckardt, in preparation

speaker	Tomoya Hayata
title	Complex Langevin simulation of rotating Bose-Einstein condensate
abstract	Our recent work on application of lattice QCD techniques to cold-atom systems is presented [1]. We study the quantum vortex nucleation in the Bose-Einstein condensate in the presence of full-quantum fluctuations. For this purpose, we perform the ab-initio simulation of rotating Bose-Einstein condensate by using the complex Langevin method, which has been developed to overcome the sign problem in the lattice QCD simulation at finite density. We simulate the nonrelativistic boson field theory at finite chemical potential under rotation. We show that in the condensed phase, vortices are generated above a critical angular velocity and the circulation is clearly quantized even in the presence of quantum fluctuations. [1] T. Hayata and A. Yamamoto, arXiv:1411.5195 [cond-mat.quant-gas].

June 4 (Thu) Ryusuke Hamajaki
    "Chaos and quantum thermalization"
   Mark Srednicki, Phys. Rev. E 50, 888