Schedule of winter semester (start from 13:00 @ #933)
|title||Many-body dynamics in a spinor BEC|
|abstract||The many-body ground state of a spinor BEC is known to be fragmented when the spin-exchange interaction is not ferromagnetic: condensation of a spin-singlet pair or a spin-singlet trimmer occurs. In this seminar, I show that the Goldstone mode in the Bogoliubov spectrum of a single condensate, i.e., a mean-field ground state, naturally includes the instability towards the fragmented ground state. However, when we take into account the effect of thermal components, the mean-field state is stabilized.|
|title||Bose physics in quantum magnets with coupled-dimer structures|
In the first part of my talk, I will review the Bose physics that emerges in quantum magnets with coupled-dimer structures . In these magnets, a local triplet excitation (triplon) behaves like a Bose particle, and such particles condense or crystallize just li
ke cold atoms on optical lattices. The second part of the talk will focus on our recent study  on a specific spin model proposed for (CuCl)LaNb2O7. In this model, antiferromagnetic dimers are ferromagnetically coupled to each other on the distort
ed Shastry-Sutherland lattice. We showed that the triplons undergo a Bose condensation or a phase separation depending on the model parameters. These are observed as a smooth increase or a jump, respectively, in the magnetization process. Implications of our results for
(CuCl)LaNb2O7 and related compounds are presented.
 T. Giamarchi, Ch. Ruegg, and O. Tchernyshyov, Nat. Phys. 4, 198 (2008).
 S.F., T. Dodds, and Y.B. Kim, Phys. Rev. B 84, 054432 (2011).
|title||Free-space coupling of a micro-mechanical oscillator to an atomic ensemble|
We discuss a hybrid quantum system where the motion of a
micro-mechanical membrane is coupled to an atomic ensemble via a
free-space laser beam. The well-developed toolbox available for the
control of the atoms enables indirect manipulation of the membrane,
such as, e.g., sympathetic cooling of its motion. Starting from a
microscopic model of the system, we derive an effective description
for the interactions between membrane and atoms, which we cast into
the form of a (partly cascaded) master-equation. We discuss major
imperfections in the system and analyze the prospects for cooling the
membrane to its quantum-mechanical ground state.
|title||Mixtures of two spin-1 condensates|
|abstract||In this talk, I will discuss related physical phenomena of binary mixtures of spin-1 Bose-Einstein condensates. Under mean-field and single-mode approximations, we have derived ground-state phase diagram, in which one interesting ground state with axisymmetry broken exists for the case of strong enough interspecies anti-ferromagnetic spin-exchange interactions. Including quantum fluctuations, the spin-dependent Hamiltonian contains noncommuting terms, as the s-wave scattering channel between interspecies does not conform to a fixed symmetry. Making use of the spin total angular momentum conservation, we numerically derive the information of the building blocks and evaluate von Neumann entropy to quantify the ground states, which show fragmented and entangled behaviors within large parameter spaces of interspecies spin-exchange and singlet-pairing interactions. Lastly, I will briefly discuss how to determine interspecies spin-exchange and singlet-pairing interactions from subsequent spin mixing dynamics, by preparing suitable initial states and properly tuning external magnetic fields.|
|title||Hydrodynamic description of spin-1 Bose-Einstein condensates|
|abstract||Properties of the spinor Bose-Einstein condensates (BEC), such as the low-lying collective modes and the ground states, have been explored extensively in the mean-field regime by solving the Gross-Pitaevskii (GP) equation. However, the GP description based on wave functions that cannot be observed directly prevents us from an intuitive understanding of the spinor BEC. In order to give a more clear-cut picture of the spinor BEC, the nematic tensor that indicates the anisotropy of the wave functions in the spin-space is introduced, and then the hydrodynamic equations in an arbitrary state of the F=1 spinor BEC are derived in terms of the density, the spin vector, and the nematic tensor. Here, the obtained equations are equivalent to the GP equation and do not involve the wave functions. By applying the single-mode approximation to these hydrodynamic equations, we reveal that the spin vector acts on the nematic tensor like torque. In this way, the hydrodynamic equations help us directly understand the physics of the spinor BEC.|
|title||Synthetic Non-Abelian Gauge Fields in Ultracold Neutral Atoms|
|title||Formulation of Uncertainty Relations between Error and Disturbance in Quantum Measurement by using Quantum Estimation Theory|
It is necessary to involve quantum estimation theory for formulating error and disturbance in quantum measurement.
We formulate the error and disturbance in quantum measurement by using the Fisher information that bounds the variance of the estimated value of 〈X〉, where 〈X〉 is the expectation value of the observable X.
We show uncertainty relations between the error of two non-commuting observables , and between the error and disturbance .
 Physical Review A 84, 042121 (2011).
 arXiv:1106.2526 (2011).
|title||Vortex core states in spin-1 Bose-Einstein condensates|
|abstract||In my talk, I introduce possible vortex core states in spin-1 Bose-Einstein condensates (BECs) with respective to a symmetry of system. The homotopy theory has been so far applied to a classification of vortices in terms of the map from the boundary of vortices to the order parameter space. However it does not determine the vortex core structures since field configurations leave from the order parameter space near the vortex core. Here, we will extend this conventional homtopy theory classification in order to deal with the core structure of vortices. To this end, we will study a map from the whole space including a core to a space of the whole degrees of freedom of an order parameter, which is lager than the order parameter space. As a result, we find many possible vortex core structures including conventional vortex cores, which have been obtained in a numerical simulation by a number of researchers.|
|title||Universal trimers and Efimov trimers|
For a three-body system interacting via resonant two-body interaction, there exist three-body bound
states called Emov states. Emov states have attracted a lot of interest since their recent experimental
realizations with ultracold atoms . One of the intriguing features of the Emov states is their universal
property: they can be characterized completely by two parameters, the s-wave scattering length and a
short-range three-body parameter, and are unaffected by all other details of the potential. Recently,
however, novel three-body bound states have been predicted theoretically , which depend only on the
s-wave scattering length. Although the origin of these trimers is closely related to the Efimov effect, they
have a distinct nature. We will discuss on the relationship between these two kinds of three-body bound
 F. Ferlaino, and R. Grimm, Physics, 3, 9 (2010)
 O. I. Kartavtsev, and A. V. Malykh, J. Phys. B, 40, 1429 (2007)
 S. Endo, P. Naidon, and M. Ueda, Few-body Systems, 51, 207 (2011)
 S. Endo, P. Naidon, and M. Ueda, in preparation
|speaker||Nguyen Thanh Phuc|
|title||Effects of quantum depletion on the energy spectrum of a ferromagnetic Bose-Einstein condensate|
In contrast to scalar Bose-Einstein condensates (BECs), in, for example, 87Rb spinor BECs, due to the large ratio of spin-independent to spin-exchange interaction, the effect of a small fraction of noncondensed atoms on the system's order parameter is remarkable and leads to a significant modification of the ground-state phase diagram . Similarly, the effect of quantum depletion on the energy spectra of the elementary excitations of a spinor BEC cannot be neglected in priori. In another aspect, for spinor BECs, the atoms posess unfrozen spin degrees of freedom, which leads to the existence of spin-wave excitations in addition to ordinary density-wave excitations in scalar Bose gases. Here, we investigate the effect of quantum depletion on the energy spectrum and propose an experimental setup to probe this effect based on the nature of spin-wave excitations.
 N. T. Phuc, Y. Kawaguchi, M. Ueda, PRA 84, 043645 (2011).
|speaker||Tatsuhiko N. Ikeda|
|title||Two thermalization mechanisms and their interplay examined in one-dimensional Bose gas|
|abstract||Thermalization is the phenomenon in which equilibrated value of an observable corresponds to the prediction of statistical mechanics. To understand thermalization in closed quantum systems, there proposed two conjectures, eigenstate thermalization hypothesis(ETH) and eigenstate randomization hypothesis(ERH). Both of these are the statements on the property of the expectation values of observables taken over each many-body eigenstate. In this seminar, we briefly review these hypotheses and examine the validities of them by invoking the Lieb-Liniger model, which describe the one-dimensional Bose gas with delta-function interaction. After the examination, we discuss the interplay between the ETH and ERH.|
|title||Theory of quantum continuous measurement in diffusive and jump type|
|abstract||The continuous quantum measurement is a quantum measurement process of continuous output in time. This measurement process can be mathematically described by stochastic master equation (SME) or stochastic Schrödinger equation (SSE) of which two types h ave been mainly discussed; jump type and dissusive type SSE's. The jump type SSE describes discrete outputs continuous measurement such as a photon counting measurement of photon field, while the measurement described by diffusive type SSE has continuous outputs such as a homodyne measurement of quadrature amplitude. In this seminar we generalize these two types of measuerement process to a measurement in which these two types of outputs exist. Then, we discuss the simultaneous measurement of photon counting and homodyne measurement as an example of this generalized process. We derive analytical expression of the time evolution of the wave function under given outputs and, using this, derive probability distribution of the two measurement outputs. This general results are applied to typical initial states: coherent states, number states, and squeezed states. These calculations shows that the correlation between the two measurement outputs comes from non-classicality of the state.|
|title||Formulation of generalized Second law with feedback control on quantum correlated states|
Quantum Maxwell's demon is a feedback controller for quantum states. This demon controls the quantum fluctuation and extracts work from the system. There has been a previous study of feedback control of the thermal fluctuation. In this work, the information content and thermodynamic variables are treated on an equal footing. We want to generalize this work on states that are quantum correlated (or entangled), and find out the appropriate quantum information content on an equal footing with thermodynamic variables. We discuss a feedback control for states that are quantum correlated and derive a generalized Second law.
 T. Sagawa and M. Ueda, Phys. Rev. Lett. 100, 80403 (2008)
|title||New Definition of Work in Thermodynamics of Small Systems|
|abstract||Thermodynamic relations of small systems such as Jarzynski equality have been studied. However, the usual definition of work doesn't mean usable work. So, we give a reasonable definition of usable work, and show a relation for this newly defined work. The relation suggests that we need extra energy kT for each operation.|
|title||Derivation and interpretation of Jarzynski equality|
|abstract||Jarzynski equality is the relation connecting the free energies of equilibrium states and the work performed during the nonequilibrium process. This equality has been proven for both Hamiltonian dynamics and stochastic dynamics. In this presentation, I will talk about the differences and the meanings of these derivations. I also discuss the critical assumptions which these derivations rely on.|
|title||Robustness of edge states in quantum walks|
|abstract||The quantum walk is a generalization of the classical random walk to quantum systems. Quantum walks can be analyzed using topology of energy bands, like topological insulators. Because of the simplicity of the model, analytic solutions can be easily obtained, for example, for edge states. In this presentation, we show that robustness of edge states in spacially inhomogeneous systems which stems from topological nature both by numerical simulation and phase diagrams.|
|title||Criteria of off-diagonal long-range order in Bose and Fermi systems based on the Lee-Yang cluster expansion method|
In this seminar, we extend the quantum-statistical cluster expansion
method of Lee and Yang  to investigate off-diagonal long-range
order (ODLRO) in one- and multi-component mixtures of bosons or
fermions . Our formulation is formally applicable to a uniform
system and a trapped system without local-density approximation and
allows systematic expansions of one- and multi-particle reduced
density matrices in terms of cluster functions which are defined for
the same system with Boltzmann statistics. Each term in this expansion
can be associated with a Lee-Yang graph. In the case of Bose
statistics, an infinite series of Lee-Yang 1-graphs is shown to
converge and gives the criteria of ODLRO at the one-particle level. In
the case of Fermi statistics, an infinite series of Lee-Yang 2-graphs
is shown to converge and gives the criteria of ODLRO at the
 T. D. Lee and C. N. Yang, Phys. Rev. 113, 1165 (1958); 117, 22 (1960)
 NS, Norio Kawakami and Masahito Ueda, arXiv:1112.5768
|speaker||Muneto Nitta (Keio University)|
|title||Majorana meets Coxeter: Novel Non-Abelian Statistics from Non-Abelian Majorana Fermions|
|abstract||First I will review non-Abelian statistics of vortices having zero-energy Majorana fermion found by Ivanov. Second, I will discuss statistics of vortices having multiple zero-energy Majorana fermions inside them (non-Abelian Majorana fermions). Considering the system of multiple non-Abelian vortices, we derive a non-Abelian statistics that differs from the one of Ivanov. The new non-Abelian statistics presented here is given by a tensor product of two different groups, namely the non-Abelian statistics obeyed by the Abelian Majorana fermions and the Coxeter group. As the simplest example, we consider the case in which a vortex contains three Majorana fermions that are mixed with each other under the SO(3) transformations. I will make a comment that such a system is so far realized in a color superconductor of high density QCD, but I will discuss a possibility that it may be realized in cold atomic gasses. This talk is based on the work with Shigehiro Yasui, Kazunori Itakura, Phys.Rev.B83:134518,2011 [arXiv:1010.3331 [cond-mat.mes-hall]].|
(a) 一粒子グリーン関数の極がBogoliubov励起で，長波長極限で無限の寿命をもち励起エネルギーが0となる (Beliaev, 1958; Hugenholtz and Pines, 1959)。
(b) 二粒子グリーン関数は一粒子グリーン関数と共通の極を持つ(Gavoret and Nozières, 1964)。
ことが知られてきた（Hohenberg and Martin, 1965; Griffin, 1996）。
 T. Kita, Phys. Rev. B 80, 214502 (2009).
 T. Kita, J. Phys. Soc. Jpn. 74, 1891 (2005); 74, 3397(E) (2005).
 T. Kita, Phys. Rev. B 81, 214513 (2010).
 T. Kita, J. Phys. Soc. Jpn. 80, 084606 (2011).