Department of Physics, The University of Tokyo
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2008 seminars

SCHEDULE (start from 13:30 @ #933)
Oct. 7 Miguel A. Cazalilla
Oct. 14 Takahiro Sagawa
Oct. 28 Peng Zhang
Nov. 4 Shi-Jie Yang
Nov. 10 (Mon.) 14:00 - 16:00 @ #233
    Christopher J. Pethick (Niels Bohr Institute)
Nov. 11 Pascal Naidon
Nov. 18 Masaki Tezuka
Nov. 25 Shuta Nakajima
Dec. 2 Muneto Nitta (Keio University)
Dec. 9 Michikazu Kobayashi
Dec. 16 Yuki Kawaguchi
Jan. 13 Koji Inokuchi
Jan. 20 Yu Watanabe
Jan. 26 (Mon.) @ Koshiba Hall
    15:00 - 16:30
    Artur Ekert (Cambridge), "Classical, quantum and post-quantum cryptography"
    16:30 - 18:00
    Tony Leggett (UIUC), "Cuprate superconductivity:the current state of play"
Jan. 27 Shingo Kobayashi
Feb. 3 Liang Tian and Xu Zhang
Feb. 23 (Mon.) 13:30 - @ #512 Sang Wook Kim
Feb. 24 Masashi Yasunaga (Osaka City Univ.)
Mar. 10 Tsutomu Momoi (RIKEN) cancelled
Mar. 17 13:00 - Toshihiro Sato (Univ. of Tokyo, ISSP)

2008/10/7(Tue) 13:30
speaker Miguel A. Cazalilla
title Pomeranchuk Instabilities in harmonically trapped Fermi gases
abstract TBA

2008/10/14(Tue) 13:30
speaker Takahiro Sagawa
title Information Thermodynamics
abstract Thermodynamics of information processing is not only a classic fundamental problem since Maxwell and Szilard have discussed, but also a modern issue in nonequilibrium statistical mechanics and quantum information theory.  In this talk, I discuss the second law of "information thermodynamics", in which the information content and thermodynamic variables are treated on an equal footing.   The second law of information thermodynamics leads to the fundamental upper bound of the work that can be extracted from a heat engine with the assistance of "Maxwell's demon", and also leads to the fundamental lower bounds of the work needed for the measurement and the erasure of information by the demon.  I will also show that, the lower bound for the erasure vindicates the "Landauer's principle" for a special case but otherwise implies its breakdown.  I will also show an explicit model of the demon's memory which violates the Landauer's principle.

2008/10/28(Tue) 13:30
speaker Peng Zhang
title The control of the scattering lengths between dressed states of fermonic atoms
abstract In the recent years, the systems with multi-component cold fermonic atoms attract many interests. The physical properties of such systems are essentially determined by the scattering lengths between different components. However, by means of the magnetic field induced Feshbach resonance, one can only independently tune one scattering length. In this work we proposed a method to realize the independent control of two (or more) scattering lengths in the multi-component system. Our proposal is a generalization of the RF induced Feshbach resonance. We show that, if the atomic hyperfine states are coupled with each other, the scattering lengths between the dressed states would depend on the Rabi frequencies and detuning of the couplings. Therefore people would obtain more control parameters, which can be used in the independent tuning of different scattering lengths.

2008/11/4(Tue) 13:30
speaker Shi-Jie Yang
title I. Creating solitons and giant vortex in the BEC
II. A quantum Monte Carlo study of Bose atoms on the dice lattice.
abstract In topic I, We present studies on the Bose-Einstein condensates through numerically solving the Gross-Pitaevskii equation. It shows that solitons can dynamically be generated in an evolving BEC. In a two-species BEC, properly arranging the s-wave scattering-length may help to create a giant skyrmion in a magnetic trap. Topic II reports some preliminary results of the quantum Monte Carlo simulations on Bose atoms on a dice lattice. The phase diagrams are briefly plotted. Several remarks on supersolid are posed. Further resolutions are still in progress.

2008/11/10(Mon) 14:00 Faculty of Science Bldg.1 #233
speaker Prof. Christopher J. Pethick (NORDITA and the Niels Bohr International Academy)
title Spin correlations in ultracold gases and collapsing stars
abstract Measurements of spin correlations in condensed matter and ultracold gases are an important probe of the microscopic state of the system. The talk will begin with an historical survey of some aspects of the problem in which the role of conservation laws will be emphasized. This is be followed by some topics related to contemporary problems, including rf spectroscopy of ultracold gases and the interaction of neutrinos with matter in collapsing stars. Similarities between ultracold Fermi gases with resonant interactions and nuclear matter at densities much below those in atomic nuclei will be explored.

2008/11/11(Tue) 13:30
speaker Pascal Naidon
title Is there an Efimov trimer in three-component lithium?
abstract In some recent experiments on ultracold lithium 6, losses of atoms by three-body collisions have been observed when the atoms are prepared in their lowest three hyperfine states in equal amounts. These specific three-body losses are thought to originate from a resonance between three colliding atoms in the three different hyperfine states and a trimer (=triatomic molecule). It has been speculated that this trimer could be a so-called "Efimov trimer", a particular kind of trimer which occurs when the two-body interactions are resonant. The purpose of our work is to determine whether the Efimov trimer theory can indeed account for the experimental observations. In this seminar, I will briefly review the Efimov trimer theory and show its application to the case of lithium.

2008/11/18(Tue) 13:30
speaker Masaki Tezuka
title Imbalanced Fermi Superfluid: Mesoscopic Enhancement of the Chandrasekhar-Clogston Limit
abstract Since the observation of condensation in population-imbalanced Fermi gases by two groups of experimentalists at MIT and Rice, the apparent discrepancy between their results has been the subject of extensive debate. The results differ over the Chandrasekhar-Clogston (CC) limit, which is the imbalance ratio where condensate disappears, and validity of local density approximation (LDA). By solving the Bogoliubov-de Gennes equations with coupling-constant renormalization appropriate for a trapped system with a chemical potential difference, we show that the CC limit increases with decreasing the atom number and increasing the trap aspect ratio. As the trap aspect ratio increases, LDA starts to be violated. This finding reconciles the apparent discrepancy.
(Reference) M. Tezuka and M. Ueda, arXiv:0811.1650.

2008/11/25(Tue) 13:30
speaker Shuta Nakajima
title Strongly interacting two-component Fermi gases and three- component Fermi gases
abstract In the first half of my talk, I will talk about our experiments of strongly interacting two-component Fermi gas. In the unitary limit at which the scattering length diverges, the gas is expected to exhibit a universal thermodynamic behavior independent of any microscopic details of the underlying interactions. We measured the sound velocity of the unitary gas to investigate the universal thermodynamic behavior. I will discuss about the experimental results. I the latter half of the talk, I will talk about our experiments of three-component Fermi gas. Recently, Ottenstein et.al. observed an unexpected loss features in the three component mixture of 6Li atoms at 130 G and found that the values of the three-body loss coefficient at 130G are enhanced by three orders of magnitude compared to the minimal value. Now we make a balanced mixture of three lowest-lying hyperfine states of 6Li atoms and plan some experiments to identify the detailed physical process of the loss features at 130 G.

2008/12/2(Tue) 13:30
speaker Muneto Nitta (Keio University)
title Non-Abelian Vortices in High Energy and Hadron Physics
abstract Five years ago, non-Abelian vortices were discovered in (supersymmetric) gauge theories and string theory independently. Those were suggested to be a key to understand confinement and duality of (supersymmetric) QCD, as a generalization of the old idea of Nambu and Mandelstam. In 2005 similar but a bit different non-Abelian vortices were found in the color-flavor locked phase (color superconductor) of dense QCD, which is expected to be realized in core of neutron stars. Furthermore, those vortices are candidates of non-Abelian cosmic strings; When two of them collide, their reconnection is inevitable. I will explain, for condensed matter theorists, those developments in these five years.

2008/12/9(Tue) 13:30
speaker Michikazu Kobayashi
title Non-Abelian vortices and their collision dynamics
abstract In this seminar, I talk about non-Abelian vortices defined as ones, the first homotopy group of which becomes non-Abelian. The non-Abelian character becomes quite remarkable when considering their collision dynamics which is defined by the non-commutative composition law of two vortices. Unlike Abelian vortices, for example, two non-Abelian vortices do neither reconnect nor passing through themselves but create a rung between them. One of the most simplest non-Abelian vortices can be realized in the cyclic phase of a spin-2 spinor Bose-Einstein condensate, and our prediction can be tested in this system.

2008/12/16(Tue) 13:30
speaker Yuki Kawaguchi
title Pattern formation dynamics in a spinor dipolar BEC
abstract In this seminar, I will discuss the pattern formation dynamics of a spinor dipolar BEC observed by Berkeley group[1]. In the experiment, a helix of the transverse magnetization develops into small domains under a uniform magnetic field. After a brief introduction of the spinor dipolar BEC, I will first show the ground state texture in the presence of the external field. Then, I will discuss the dynamics. Though the Bogoliubov analysis suggests that the initial state is dynamically unstable, neither the time scale nor the wave length of the instability match the experiment.
[1] M. Vengalattore, et. al., PRL 100, 170403 (2008)

2009/1/13(Tue) 13:30
speaker Koji Inokuchi
title Quantum Cluster Expansion Approach for BEC-BCS Crossover
abstract We theoretically study the BEC-BCS crossover in an ultracold Fermi gas by means of Lee-Yang quantum cluster expansion [1]. This method is a quantum statistical cumulant expansion that enables us to systematically expand the grand partition function in terms of binary kernel, which exactly describes two-body scattering. In particular, this method is useful in the system where the interaction can be characterized by a scattering length, such as ultracold gases. Using pseudo-potential for the inter-atomic interaction, we sum up the contribution of the ladder diagrams in the quantum cluster expansion, which directly relates to the Cooper instability. The critical temperature can be defined as the divergent point of the grand partition function. The obtained critical temperature agrees with the BEC transition temperature for dimmers in the BEC limit and with the Thouless criterion in the BCS limit.
[1] T. D. Lee and C. N. Yang, Phys. Rev. 113, 1165 (1958)

2009/1/20(Tue) 13:30
speaker Yu Watanabe
title Optimal Measurement and Maximal Fisher Information
abstract For estimating an expectation value of an observable for an unknown quantum state, we believe that the projection measurement for the observable is an optimal measurement. However, what is the definition of optimality ? I define that an optimal measurement is the measurement maximize the Fisher information about the observable. Then I probe that the projection measurement for the observable is an optimal measurement for any finite N-dimensional quantum system, and I calculate the maximal fisher information by the measurement.

2009/1/27(Tue) 13:30
speaker Shingo Kobayashi
title Classification of defects and homotopy theory
abstract Defects in ordered media is well classified by the homotopy classes in order parameter manifold. I'm going to explain a general definition of homotopy and apply the homotopy theory to condensed matter physics for the classification of topological defects, According to N.D.Mermin[1].
[1]N.D.Mermin 1979 Rev.Mod.Phys.51 591

2009/2/3(Tue) 13:30
speaker Liang Tian
title BCS-BEC Crossover
abstract I will first talk about zero temperature BCS-BEC crossover by A.J Leggett and its extension to finite temperature BCS-BEC crossover by P.Nozières and S.Schmitt-Rink (NSR). Even though BCS limit and BEC limit can be explained by NSR's approach, this method cannot explain the crossover region correctly, for only ladder diagrams are considered and the effects of medium are not taken into account. By taking into account the effects of medium, some additional terms, which were not included in NSR's original results, appear in the equations that determine transition temperature and chemical potential.

2009/2/3(Tue) 13:30
speaker Xu Zhang
title Kosterlitz-Thouless Transition
abstract In this seminar, I will talk about one special phase transition in two dimensional systems which is called Kosterlitz-Thouless transition. I will first talk about the critical dimension which relates to the question that why no ordinary phase transition occurs in two dimensional continuous systems. Then I will use the planar spin model to explain the KT transition and topological defect that is responsible for this transition. I will also introduce the renormalization group theory that is used for the further analysis of KT transition. Finally I will discuss about KT transition in the systems with spin degrees of freedom.

2009/2/23(Mon) 13:30 @ #512
speaker Sang Wook Kim
title Quantum Szillard engine revisited: decoherence driven atomic transport
abstract I propose a new mechanism to generate a dc current of particles without any chemical potential difference based upon a noble interplay between coherence and decoherence. I show that the dc current arises if the transport process in one direction is maintained coherent while the process in the opposite direction is incoherent. I provide possible implementations of the idea using an atomic Michelson interferometer and a ring interferometer. The physical mechanism is discussed in the context of the violation of the second law of thermodynamics (Maxwell's demon) and the detailed balance. The relation to Scully's quantum Carnot engine and Raizen's atom diode are also mentioned.

2009/2/24(Tue) 13:30
speaker Masashi Yasunaga
title Magnetic Resonance in Spinor Bose-Einstein Condensates
abstract I introduce magnetic resonance (MR) phenomena in atomic gaseous spin-1 Bose-Einstein condensates, discussing numerical results of Gross-Pitaevskii equations and prospects of the resonance. First, I demonstrate spin echo, which is a pulse sequence refocusing a coherence of spins, in a gradient magnetic field. The echo peaks shows information of a Stern-Gelach phase separation in the condensates. Next, explaining a relation between Josephson effects and quadratic Zeeman effects by using a single spatial mode approximation, I report the transition from Rabi oscillations to AC Josephson oscillations. These MR phenomena represent hallmarks in the condensates.

2009/3/17(Tue) 13:00
speaker Toshihiro Sato
title Numerical approaches for thermal dynamics in bosonic systems
abstract Since the experimental achievement of Bose-Einstein condensation in dilute atomic gases, it has been possible to study dynamical properties of collective modes, vortex nucleation and so on from both experimental aspects and theoretical aspects. At very low temperature, these dynamics properties have been successfully modeled by Gross-Pitaevskii equation. On the other hands, it is difficult to numerically treat thermal dynamics in bosonic systems and how it is treated is a demanded problem. Recently, Davis et al. developed projected Gross-Pitaevskii equation to treat thermal dynamics in bosonic systems [1]. In this seminar, I will briefly introduce about details of this method and demonstrate thermal dynamics of a dilute Bose gas in the harmonic trap.
[1] M. J. Davis, R. J. Ballagh, and K. Burnett, J. Phys. B 34, 4487 (2001).