Department of Physics, The University of Tokyo
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2012 seminars

Schedule of winter semester (start from 13:00 @ #933)
Oct. 4 Cord Mueller (Centre for Quantum Technologies, Singapore)
Oct. 11 Shunsuke Furukawa
Oct. 18 (no seminar)
Oct. 25 Zhifang Xu
Nov. 1 Naoyuki Sakumichi
Nov. 8 Shohei Watabe
Nov. 15 Shingo Kobayashi
Nov. 22 Shinpei Endo
Nov. 29 Nguyen Thanh Phuc
Dec. 6 Tatsuhiko Ikeda
Dec. 13 Yui Kuramochi
Dec. 20 Ken Funo
Jan. 10 (no seminar)
Jan. 17 Yusuke Horinouchi
Jan. 24 Yasuhiro Hatsugai (Univ. of Tsukuba)
Feb. 14 Sho Sugiura (Dept. of Basic Science, Univ. of Tokyo)


2012/10/4(Thu) @#933 13:00-
speaker Cord Mueller (Centre for Quantum Technologies, Singapore)
title Bogoliubov theory of disordered Bose-Einstein condensates
abstract When interacting bosons are condensed in optical potentials of various forms, it can be quite a challenge to tell the condensate from its excitations, both quantum and thermal. In this talk, I will describe a Bogoliubov theory of inhomogeneous condensates that is capable of describing the excitations of bosonic superfluids in arbitrary external potentials.
Joint work with Christopher Gaul (Madrid).
Recent reference: arXiv:1202.3489

2012/10/11(Thu) @#933 14:00-
speaker Shunsuke Furukawa
title Quantum Hall states in rapidly rotating two-component Bose gases
abstract Under rapid rotation, ultracold gases of bosonic atoms have been predicted to enter a highly correlated regime, which is analogaous to quantum Hall systems. In this talk, I will present our recent study on the quantum Hall states in rapidly rotating two-component (or pseudo-spin-1/2) Bose gases. These systems offer an ideal setting in which to study the roles of "(pseudo-)spins" in quantum Hall physics. Our main results include (a) the numerical evidence for a non-Abelian spin-singlet state, whose quasiparticles feature non-Abelian statistics, and (b) a phase transtion between different quantum Hall states that occurs as the ratio of the intercomponent to intracomponent interactions changes.
Reference: S.F. and M. Ueda, Phys. Rev. A 86, 031604(R) (2012)

2012/10/25(Thu) @#933 13:00-
speaker Zhifang Xu
title Spinor Bose-Einstein condensates with spin-orbit couplings
abstract Recently, the groundbreaking experiments in Spielman's group on emulating spin-orbit couplings in a pseudo spin-1/2 atomic Bose gases have stimulated tremendous efforts on spin-orbit coupled quantum gases. Based on mean-field approximations, plane-wave, stripe, triangular-lattice, and square-lattice phases are found as ground states. In this talk, I will present our recent study on how to systematically classify different phases based on their symmetries. We can then not only understand different lattice phases already found but also find two different kagome lattice phase and a nematic vortex lattice phase, both of which emerge spontaneously without lattice potentials.
Reference: Z. F. Xu, Y. Kawaguchi, L. You, and M. Ueda, Phys. Rev. A 86, 033628 (2012).

2012/11/1(Thu) @#933 13:00-
speaker Naoyuki Sakumichi
title BEC-BCS crossover theory based on Lee-Yang cluster expansion
abstract We propose a new systematic approach to the BCS-BEC crossover at finite temperature based on the cluster (virial) expansion formulation of Lee and Yang [1], which enables us to systematically expand the thermodynamic function in terms of cluster functions. It is found that the proposed theory leads the thermodynamic function of a standard BCS-BEC crossover theory of Noziéres and Schmitt-Rink (NSR) [2] by first-order approximation. Our approach is basically different from standard perturbative-expansion techniques in terms of interaction parameter, as the theory of NSR. Concretely, although the second virial coefficient (which is dominant in the high-temperature limit) of NSR is equivalent to that of ideal Fermi gas, that of the proposed theory is exact for any value of an s-wave scattering length.
[1] T. D. Lee and C. N. Yang, Phys. Rev. 113, 1165 (1958); 117, 22 (1960).
[2] P. Noziéres and S. Schmitt-Rink, J. Low Temp. Phys., 59, 195 (1985).

2012/11/8(Thu) @#933 13:00-
speaker Shohei Watabe
title recent development of Monte Carlo methods
abstract The Green's function formalism is one of useful tools for studying many-body systems. This is a perturbation theory, and if one finds minimum diagrams describing the system he is considering qualitatively or quantitatively well, it helps him understand what mechanism might be there. On the other hand, another interesting tool is known for studying condensed matters; that is the Monte Carlo method. This method gives unbiased results for some systems. In this seminar, I will review recent development of Monte Carlo methods, and talk about what I am going to do.

2012/11/15(Thu) @#933 13:00-
speaker Shingo Kobayashi
title Topological influence and back-action in multiple topological excitation systems
abstract Topological excitations exist in various subfields of physics, such as condensed matter physics, elementary particle physics, and cosmology. They have been observed experimentally in gaseous Bose-Einstein condensates. We can classify them using the homotopy group. However, there is a case that the homotopy group is not consistent with a charge of a topological excitation when it coexists with a vortex, which effect is called the topological influence. In this case, physically consistent charges are given by the Abe homotopy group [1,2]. In this talk, I will discuss the relationship between the topological influence and the total charge conservation. To be consistent with the charge conservation, I introduce a back-action on a vortex in terms of the topological influence on a topological excitation.
[1] M. Abe, Jpn. J. Math. 16, (1940) 179.
[2] S. Kobayashi, et al., Nucl. Phys. B 856, (2012) 577.

2012/11/22(Thu) @#933 13:00-
speaker Shimpei Endo
title Polarons in a mass imbalanced two component Fermi gas
abstract Recently, mass imbalanced atomic mixtures such as K-Li, Yb-Li, Cs-Li, have been realized experimentally. In these systems, non-trivial three-body or four-body bound states can appear and their interplay with many-body physics is of great interests. Furthermore, the heavy mass particle can be used as a probe in a Fermi environment.
In this seminar, I discuss the polaron physics in this mass imbalanced Fermi gas. I discuss the dynamics of a single heavy particle immersed in a Fermionic environment. I then talk about multi-heavy particles immersed in a Fermionic environment, and their effective interactions.

2012/11/29(Thu) @#933 13:00-
speaker Nguyen Thanh Phuc
title Fluctuation-induced first-order quantum phase transition in spinor Bose-Einstein condensates
abstract Quantum fluctuations are ubiquitous in a wide range of physical phenomena. In ultracold atoms it has also been proven to play an important role in the breaking of symmetry. In spin-2 spinor Bose-Einstein condensates, the energy spectrum contradicts with the order of phase transition at the mean-field level.
In this seminar, by taking quantum fluctuations into account, we show that the energy spectrum reconciles with the first-order quantum phase transition. The energy gap of quasi-Nambu-Goldstone modes and Beliaev damping are also discussed.

2012/12/6(Thu) @#933 13:00-
speaker Tatsuhiko Ikeda
title Universal quantum correction to diagonal entropy after control
abstract The diagonal entropy has recently been proposed to describe the thermodynamic entropy in isolated quantum systems under control [1]. Unlike von Neumann's entropy, it varies every time we control the system. Although the second law of thermodynamics is expected to hold, it is proven under the condition that the density matrix before control is diagonal in the eigenenergy basis. To examine the second law in more general situations, without assuming the above condition, we have evaluated the diagonal entropy after control and found that it involves a universal quantum correction which is sub-extensive [2]. As a consequence, we conclude that the second law is retained in large systems but may breaks down in small systems.

[1] A. Polkovnikov, Annals of Physics 326, 486 (2011).
[2] T. N. Ikeda, A. Polkovnikov, and M. Ueda, in preparation.

2012/12/13(Thu) @#933 13:00-
speaker Yui Kuramochi
title Theory of simultaneous continuous measurement process of photon counting and homodyne detection
abstract The quantum continuous measurement is the quantum measurement in which weak quantum measurements are done continuously in time. Among the studies on the quantum continuous measurement, there are two types of measurement process: jump type and diffusive type continuous measurements.
In this seminar, we found the general theory which can include both jump type and diffusive type measurement outcomes. This theory is applied to a typical measurement process of simultaneous measurement of photon counting and homodyne detection. The stochastic Schrödinger equation discribing the measurement process is analytically solved. Using this solution, a probablity density and a generating functional of the measurement outputs are obtained. These formulae are applied to typical initial conditions: coherent, number, thermal and squeezed states. Finally Monte Carlo Simulations of the photon number expectation value for several paths are presented.

2012/12/20(Thu) @#933 13:00-
speaker Ken Funo
title Thermodynamic energy gain from entanglement
abstract When we consider an observer that can measure the microscopic degrees of freedom and feedback control the system, we can extract work from the system above the limit of the conventional second law of thermodynamics. Generalized second law by controlling thermal fluctuation has been shown. Also, information to free energy conversion has been experimently demonstrated. In this seminar, we discuss the effect of entanglement when considering the feedback control. We show that work can be extracted from the entangled system beyond the classical correlation.

2013/1/17(Thu) @#933 13:00-
speaker Yusuke Horinouchi
title Introduction to the Functional Renormalization Group
abstract The Functional Renormalization Group (FRG) is a powerfull methodology in field theory, which is applicable even in the strong coupling regime. The basic idea of the FRG is to introduce an infrared cutoff to the free propagator of the theory and to study the flow of the effective action by changing this cutoff. Because the resulting equation is exact, it has many advantages over other perturbative or non-perturbative approaches. It is a systematic non-perturbative approach which is independent of the details of the system. I will review the basic formulation of the FRG, and present its demonstration in a harmonic oscillator perturbed by a quartic interaction.

2013/1/24(Thu) @#933 13:00-
speaker Yasuhiro Hatsugai (Institute of Physics, University of Tsukuba)
title Symmetry and order parameters for topological phases
abstract Beyond the great success of the Ginzburg-Landau theory associated with symmetry breaking (SB), condensed matter physicists focus on more than that recently. That is, phases without any fundamental SB but possessing characteristic feature are of the central interest, which are the quantum/spin liquids. This class of matter includes quite wide varieties such as quantum (spin) Hall states, gapped quantum spins, anisotropic superfluids/superconductors and graphene. Some of cold atoms and photonic systems belong to the class as well. Even though the SB is absent, too much generic states are boring. Then still the symmetry plays an important role to constrain the physical states. Gauge symmetries, time-reversal and charge conjugation are important examples. When the quantum/spin liquid is stable against for some perturbation due to geometrical constraints, one may consider the state topological. As for the topological phases with some symmetry protection, we can define "topological order parameter" using topological objects, such as gap nodes, quantized Berry phases and Chern numbers. Some of edge states, which are induced by geometric perturbation such as boundaries and impurities, are again topologically stable and used for the topological order parameters. This is the bulk-edge correspondence. As for the gapped cases, these topological order parameters are adiabatic invariants and useful for identification of the quantum phase transition. We will describe generic idea of the topological phases and show validity of our topological characterization for various quantum phases.

2013/2/14(Thu) @#201a 13:00-
speaker Sho Sugiura (Dept. of Basic Science, Univ. of Tokyo)
title Thermal Pure Quantum State Corresponds To Various Ensembles
abstract A thermal equilibrium state of a quantum many-body system is conventionally represented by the density operator of the statistical-mechanical ensembles. It can also be represented by a typical pure state, which we call a thermal pure quantum (TPQ) state. This state is not obtained by purification of a mixed state, hence any extra systems such as reservoirs are unnecessary. A single realization of the TPQ state suffices for calculating all statistical-mechanical properties, including correlation functions and genuine thermodynamic variables, of a quantum system at finite temperature. In this talk, we firstly introduce the TPQ state corresponding to microcanonical ensenble. Then, we extend it to the TPQ state corresponding to canonical ensenble. The TPQ states corresponding to other ensembles can also be constructed in a similar manner. Next, We show that all these TPQ states are equivalent, i.e., they give identical thermodynamic results. We also show that they are transformed to each other by simple analytic transformations. This formulation is not only interesting as fundamental physics but also advantageous in practical applications because one needs only to construct a single pure state by just multiplying the hamiltonian matrix to a random vector.

Schedule of summer semester (start from 13:00 @ #933)
2012/4/12(Thu) @933 13:30-
speaker Shohei Watabe (Univ. of Tokyo)
title Many-body effect on an interacting Bose gas
abstract In this seminar, I will talk about our recent challenges to fix problems in mean-field theories for bosons. A well-known mean-field theory for a Bose-Einstein condensate is the Shono-Popov approximation, including all possible first-order diagrams of a Bose-condensed system. However, this theory has problems: (1) The critical temperature is equal to an ideal Bose gas. (2) Correlation functions have infrared divergence. (3) It involves the first-order phase transition. These problems are shared with the Nozieres-Schmitt-Rink theory (applicable to superfluid Fermi gases) in the BEC limit as well. I will discuss how to fix two of the three problems in Green's function language and also discuss one problem still remains. This work is collaborated with Prof. Yoji Ohashi.

Apr. 19(Thu) Shingo Kobayashi (in Japanese)
    "Chern Numbers, Quaternions, and Berry's Phases in Fermi Systems"
    J. E. Avron, L. Sadun, J. Segert and B. Simon, J. Math. Phys. 124, (1989) 595-627

Apr. 26(Thu) Shinpei Endo (in English)
    "Resonantly paired fermionic superfluids"
    V. Gurarie and L. Radzihovsky, Ann. Phys. 322 (2007) 2-119

May. 24(Thu) Nguyen Thanh Phuc (in English)
    "Dynamics of Trapped Bose Gases at Finite Temperatures"
    Zaremba Nikuni Griffin, Journal of Low Temperature Physics 116 277 (1999)

May. 31(Thu) Tatsuhiko Ikeda (in Japanese)
    "Rigorous results on valence-bond ground states in antiferromagnets"
    Affleck Kennedy Lieb Tasaki PRL 59 799-802 (1987)
    "Valence bond ground states in isotropic quantum antiferromagnets"
    Affleck Kennedy Lieb Tasaki CMP 3 477-528 (1988)

Jun. 7(Thu) Yui Kuramochi (in Japanese)
    "An Operational Approach to Quantum Probability"
    E. B. Davies and J. T. Lewis, Commun. math. Phys. 17 239-260 (1970)

Jun. 14(Thu) Ken Funo (in English)
    "Information causality as a physical principle"
    M. Pawlowski, T. Paterek, D. Kaszlikowski, V. Scarani, A. Winter, and M. Zukowski, Nature 461, 1101 (2009)

Jun. 21(Thu) Tomohiro Shitara (in Japanese)
    "Can Quantum-Mechanical Description of Physical Reality Be Considered Complete?"
    A. Einstein, B. Podolsky, and N. Rosen, Phys. Rev. 47, 777-780 (1935)
    "Can Quantum-Mechanical Description of Physical Reality be Considered Complete?"
    N. Bohr, Phys. Rev. 48, 696-702 (1935)

2012/6/25(Mon) @233 10:00-
speaker Hitoshi Murayama (IPMU & Berkeley)
title Unified description of Nambu--Goldstone bosons without Lorentz invariance
abstract Using the effective Lagrangian approach, we clarify general issues about Nambu--Goldstone bosons without Lorentz invariance. We show how to count their number and study their dispersion relations. Their number is less than the number of broken generators when some of them form canonically conjugate pairs. The pairing occurs when the generators have a nonzero expectation value of their commutator. For non-semi-simple algebras, central extensions are possible. Underlying geometry of the coset space in general is partially symplectic.

Jun. 28(Thu) Hiroyuki Shimizu (in Japanese)
    "The topological theory of defects in ordered media"
    N. D. Mermin, Rev. Mod. Phys. 51, 591-648 (1979)

2012/7/3(Tue) @431 13:00-
speaker Motohiko Ezawa (University of Tokyo)
title From graphene to silicene: A topological insulator made of silicon
abstract Silicene is a monolayer of silicon atoms forming a two-dimensional honeycomb lattice, which shares almost every remarkable property with graphene. The low energy dynamics is described by Dirac electrons, but they are massive due to relatively large spin-orbit interactions. I will explain the following properties of silicene: 1) The band structure is controllable by applying an electric field. 2) Silicene undergoes a phase transition from a topological insulator to a band insulator by applying external electric field. 3) The topological phase transition can be detected experimentally by way of diamagnetism. 4) There is a novel valley-spin selection rules revealed by way of photon absorption. 5) Silicene yields a remarkably many phases when the exchange field is additionally introduced. 6) A silicon nanotubes can be used to convey spin currents under an electric field.

Jul. 5(Thu) Shun Tanaka (in Japanese)
    "On the Einstein Podolsky Rosen paradox"
    J. S. Bell, Physics 1 (3): 195-200 (1964)
    "The Problem of Hidden Variables in Quantum Mechanics"
    Kochen, S. and Specker, Journal of Mathematics and Mechanics (1967)

2012/7/9(Mon) @201a 13:00-
speaker Takahiro Sagawa (Kyoto University)
title How to Reconcile Maxwell's Demon with the Second Law?
abstract As is known from the nineteenth century, Maxwell's demon can adiabatically decrease the entropy of thermodynamic systems by feedback control. What reconciles the demon with the second law? In this talk, I will answer this question on the basis of my recent work with Prof. Ueda (arXiv:1206.2479): the positive entropy production during measurement compensates for the negative entropy production during feedback control. This talk is organized as follows. First, I will introduce the basic concepts in information theory and nonequilibrium thermodynamics. Second, I will briefly review the history of Maxwell's demon, and clarify what was understood and what was misunderstood. Third, I will talk about our recent results on the generalizations of the fluctuation theorem for information exchanges, which clarifies the information-entropy balance in a broad class of information processing including the conventional Maxwell's demon.

Jul. 12(Thu) Yuske Horinouchi
    "Energetics of a strongly correlated Fermi gas"
    S. Tan, Ann. Phys. 323 2952 (2008)
    "Large momentum part of a strongly correlated Fermi gas"
    S. Tan, Ann. Phys. 323 2971 (2008)
    "Generalized virial theorem and pressure relation for a strongly correlated Fermi gas"
    S. Tan, Ann. Phys. 323 2987 (2008)

2012/8/27(Mon) @933 13:00-
speaker Doerte Blume (Washington State Univ.)
title Two-Component Fermi Gases with Unequal Masses:Three-, Four- and Many-Body Physics
abstract Weakly-bound few-body systems have been studied extensively by the atomic, nuclear and condensed matter communities since the early days of quantum mechanics. This talk summarizes our recent theoretical studies of few-fermion systems consisting of three and four particles. In particular, we discuss the energetics and structural properties of extremely weakly-bound three- and four-fermion systems consisting of a majority of heavy fermions and a single light impurity. For positive interspecies s-wave scattering length and sufficiently large mass ratio, a weakly-bound universal four-body bound state is predicted to exist. We also discuss the behavior of two-component Fermi gases with infinitely large interspecies s-wave scattering length. Employing the virial equation of state, thermodynamic properties of unequal-mass Fermi gases at unitarity are discussed in the high-temperature limit.

2012/8/27(Mon) @933 15:00-
speaker Jose D'Incao (JIRA, University of Colorado and NIST)
title Efimov physics for atoms and dipolar species
abstract Strides made by the field of theoretical atomic physics have resulted in a tremendous deepening of our understanding of ultracold gases in the quantum mechanical realm. Increasingly, these gains are being translated into prospects for controlling atomic behavior, whether for development of the next generation of atomic clocks, or for creating novel phases of atomic gases, or for the manipulation of chemical reaction dynamics. In this talk, I will show that a more fundamental phenomena, known today as Efimov physics, controls the interactions between a few atoms and molecules. Predicted about 40 years ago, the Efimov effect is one of the most counterintuitive quantum phenomena that manifest in a "simple" few-particle system. I will discuss our recent findings on Efimov physics in atomic systems as well as its extention to strongly dipolar systems. Even though a long-range anisotropic dipolar interaction has all the ingredients to "destroy" the Efimov effect, our work shows that not only does the effective attractive interaction that characterizes the Efimov effect persist, but also that the dipolar interaction is extremely beneficial for the study of the Efimov effect.