Department of Physics, The University of Tokyo
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2007 seminars

4/11 (wed) : M. A. Cazalilla (DIPC, Spain)
5/14 (Mon) : T. Matsuura (ECT, Italy)
10/2 (tue) : H. Terashima (PD)
10/4 (thu) : I. Danshita (NIST), R. Kanamoto (Univ. Arizona)
10/9 (tue) : T. Sagawa (M2)
10/16 (tue) : M. Tezuka (PD)
10/24 (wed) : Y. Kawaguchi (RA)
11/6 (tue) : S. Nakajima (M2)
11/20 (tue) : D. Dickerscheid (PD)
11/27 (tue) : K. Inokuchi (M1)
11/29 (thu) : P. D. Drummond (ARC Centre of Excellence for Quantum-Atom Optics)
12/4 (tue) : Y. Watanabe (M1)
12/11 (tue) : K. Totsuka (B4), S. Fujisawa (B4)
1/29 (tue) : Y. Yanase (Univ. Tokyo)
2/19 (tue) : H. Takeuchi (Osaka City Univ.)


2008/2/19(Tue) 13:00   (at S5 #411)
speaker Hiromitsu Takeuchi (Osaka City University)
title Boojums in Rotating Two-Component Bose-Einstein Condensates
abstract A boojum is a topological defect that can form only on the surface of an ordered medium such as superfluid 3He and liquid crystals. We study theoretically boojums appearing between two phases with different vortex structures in two-component BECs where the intercomponent interaction is repulsive in one phase and attractive in the other.[1] The details will be discussed in the seminar.

[1] H. Takeuchi and M. Tsubota, J. Phys. Soc. Jpn. 75, 063601 (2006).

2008/1/29(Tue) 13:00   (at S5 #411)
speaker Dr. Youichi Yanase (University of Tokyo)
title Magnetism and Superconductivity without Inversion Symmetry
abstract I am planning to talk about some characteristic properties in non-centrosymmetric superconductors which have no inversion symmetry in the crystal structure. The breakdown of inversion symmetry induces (i) the admixture of spin singlet and spin triplet order parameters, (ii) helical superconductivity, (iii) magneto-electric effect, (iv) anomalous paramagnetic effect, and so on. Our microscopic theory on the non-centrosymmetric heavy fermion materials, namely CePt_3Si, CeRhSi_3 and CeIrSi_3 will be reviewed. An applicability to the cold fermion atomic gas will be discussed.

2007/12/11(Tue) 13:00   (at S5 #411)
speaker Ken-ichiro Totsuka (B4)
title Gate operation for quantum computer in optical lattice
abstract Recently, experiments have demonstrated entanglement among an ensemble of atoms confined in optical lattice. For physical realization of quantum computation, it is important to demonstrate entanglement of key operation such as CNOT. For the purpose of my study of implementing key operations in optical lattice, I read two relevant papers; [1] [2]. In this talk, I will introduce them and plan toward graduation thesis.

[1] M. Anderlini, P.J.Lee and et al. ,Nature 448 (2007) 452-456
[2] G. K.Brennen, I. H. Deutsch, Phys. Rev. Lett. 82 (1999) 1060-1063
speaker Shinsuke Fujisawa (B4)
title Quantum feedback control of the photon number in a superconducting cavity
abstract The state collapse by measurement of a microscopic system is a main feature of quantum theory. A Recent experiment [1] observes a step-by-step collapse of the state by quantum non-demolition measurement of the photon number of a field in a superconducting cavity. Since this cavity has a extremely long damping time, there is possibility of controlling the photon number by using quantum feedback method. Here the topics of my talk are a short review of the recent experiment and the way how to control the photon number.

[1] C. Guerlin, M. Brune, S. Haroche et al., Nature 448 (2007) 889.

2007/12/4(Tue) 13:00   (at S5 #411)
speaker Yu Watanabe (M1)
title Quantification of entanglemrnt and visualization of information on qubit system
abstract Many type of quantification of entanglement are known but no one can be applicable to more than two qubit system. During processes of making entanglement on qubits which originally separable, information about original states move from one qubit to the other or to entanglement. In this talk I will visualize these information as 3D objects using accuracy parameter and discuss the relation between an efficiency of making entanglement by unitary operations and quantification of entanglement.

2007/11/29(Thu) 14:00   (Engineering Building #9, large meating Room 1F, Univ. of Tokyo)
speaker Prof. Peter D. Drummond (ARC Centre of Excellence for Quantum-Atom Optics)
title Universal thermodynamics of strongly interacting Fermi gases
abstract Experiments on ultra-cold Fermi gases at micro-Kelvin temperatures are revolutionizing many areas of physics. Their simplicity allows tests of many-body theory in areas long thought to be inaccessible. The most exciting development is the strongly interacting regime. In this limit, the gas is expected to exhibit a universal thermodynamic behaviour, at all temperatures, independent of any microscopic details of the underlying interactions. Universality means, for example, that we can obtain insight about the interior of neutron stars from table-top low-temperature experiments. Substantial experimental efforts have been carried out to verify the existence of universality. These ground-breaking investigations now provide a precise measurement, accurate to the level of a few percent. This is an exceptional accuracy in this challenging field of ultra-low temperature physics. The colloquium will outline the concept of universality, and compare a UQ-developed theory with the latest experimental data.

2007/11/27(Tue) 13:00   (at S5 #411)
speaker Kohji Inokuchi (M1)
title Toward the Bose-Einstein Condensation of Molecules
abstract We study the equilibrium state of interacting fermions in the so-called BEC regime of the BEC-BCS crossover (where the scattering length is positive and small). Experimentally, a BEC of molecules has been realized. The goal of this research is to explicitly take into account many-body effects using the "Lee-Yang cluster expansion" to investigate the formation of molecules and their subsequent condensation. By using this method, we can avoid a complexity in Fermi-Dirac statistics and describe physical quantities in terms of Boltzmann statistics [1, 2].

[1] T. D. Lee and C. N. Yang, Phys. Rev. 113, 1165 (1958)
[2] T. D. Lee and C. N. Yang, Phys. Rev. 117, 22 (1959)

2007/11/20(Tue) 13:00   (at S5 #411)
speaker Dennis Dickerscheid (PD)
title Heteronuclear fermionic superfluids with spin degrees of freedom
abstract In this talk I will discuss the theory of spinor superfluidity in a two-species heteronuclear gas consisting of ultracold fermionic atoms. I will show that, at the mean field level, these gases are very similar to spinor Bose-Einstein condensates and argue that this predicts interesting many-body physics.

2007/11/6(Tue) 13:00   (at S5 #411)
speaker Shuta Nakajima (M2)
title Crossover from a Molecular Bose-Einstein Condensate to a Resonance Condensation of Fermionic Atom Pairs
abstract Ultracold quantum gases of fermionic atoms with tunable interactions offer the possibility to study strongly correlated systems. Magnetic-field Feshbach resonances provide the means for controlling the strength of cold atom interactions characterized by the s-wave scattering length a from effectively repulsive (a>0, BEC side) to attractive (a<0, BCS side). We started experiments in 2006, and succeeded in all-optical production of Molecular Bose-Einstein Condensate(BEC) of 6Li atoms in February 2007. Now we are investigating strongly interacting molecular BEC by using "the bragg spectroscopy". In the first part of the talk I will provide brief explanations about ultracold quantum gases of atoms with tunable interactions. In the second part of the talk I will talk about bases of our experiments: the laser cooling and the bragg spectroscopy. Lastly, I will report our experimental results.

2007/10/24(Wed) 13:00   (at S5 #411)
speaker Yuki Kawaguchi (RA)
title Knot Soliton in a Spinor BEC
abstract The rich order parameter space of multicomponent BECs admit various kinds of topological excitations, beyond the simple vortices in single-component BECs. In this talk, I will show that antiferromagnetic spin-1 BECs allow for a three dimensional coreless monopole, so-called knot soliton, which corresponds to class 1 of the 3rd homotopy group (π3(S2)=Z). Such topological excitations are expected to be formed by rapid cooling of a thamal gas or external disturbance. However, the energy of the knot soliton is proportional to the size of the knot, and therefore it shrinks and collapses. I will discuss the collapse dynamics and show that the antiferromagnetic order breaks locally and local magnetization emerges, which can be an experimental signature to detect a knot soliton.

2007/10/16(Tue) 13:00   (at S5 #411)
speaker Masaki Tezuka (PD)
title Superfluidity in Fermi Gas with Population Imbalance
abstract Recent observations of superfluidity in cooled fermionic atoms (6Li) [1][2] with unequal populations of two hyperfine states have attracted renewed attentions to superfluidity of population- imbalanced fermions, where two main issues are whether superfluidity disappears at a particular value (the Chandrasekhar-Clogston (CC) limit) of population imbalance, and in what parameter regime the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase emerges. The existence of the CC limit is currently under controversy, while the FFLO phase remains elusive.
Here we study [3] a harmonically-trapped imbalanced Fermi superfluid based on a one-dimensional attractive Hubbard model, using the density-matrix renormalization group. The obtained density profile shows a flattened population difference of spin-up and spin-down components, with periodic modulations with a small amplitude, at the center of the trap. The two-particle pair correlation reveals that the sign of the order parameter changes in space with the period depending on population imbalance P=(Nu-Nd)/(Nu+Nd), demonstrating the realization of the FFLO phase.

[1] M.W. Zwierlein et al., Science 311 (2006) 492.
[2] G.B. Partridge et al., Science 311 (2006) 503.
[3] M. Tezuka and M. Ueda, arXiv:0708.0894 (preprint).

2007/10/9(Tue) 13:00   (at S5 #411)
speaker Takahiro Sagawa (M2)
title Upper Bound on our Knowledge about Noncommuting Observables for a qubit system
abstract I will present about the uncertainty relation between measurement errors of noncommuting observables. It is based on the characterization of the quantum-measurement accuracy in terms of a 3 by 3 matrix. This matrix, which we refer to as the accuracy matrix, can be calculated from a positive operator-valued measure (POVM) corresponding to the quantum measurement. The uncertainty relation offers a quantitative information-theoretic representation of Bohr's principle of complementarity, and can be interpreted as a trade-off relation on the asymptotic accuracy of the maximum-likelihood estimation of the probability distributions of observables.

2007/10/4(Thu) 13:00   (at S5 #411)
speaker Dr. Ippei Danshita (NIST)
title Quantum phases of ultracold bosons in double-well optical lattices
abstract Recently, the experimental realization of double-well optical lattices has attracted much interest [1]. In this talk, I will discuss the superfluid and insulating phases of bosons in double-well optical lattices and focus on the specific example of a two-legged ladder, which is currently accessible in experiments. Applying both mean-field and time-evolving block decimation [2] techniques to the two-leg Bose-Hubbard Hamiltonian, the zero-temperature phase diagrams are obtained. The critical points separating the insulating and superfluid phases at commensurate fillings, where the Berezinskii-Kosterlitz-Thouless transition occurs, are determined. I will show that the phase diagram depends significantly on the interchain hopping and tilt between double wells. In particular, the insulating phase at unit filling exhibits a non-monotonic behavior as a function of the tilt parameter, producing a reentrant phase transition between superfluid and insulating phases.

[1] J. Sebby-Strabley et al., Phys. Rev. A 73, 033605 (2006); M. Anderlini et al., Nature (London) 448, 452 (2007).
[2] G. Vidal, Phys. Rev. Lett. 93, 040502 (2004); ibid. 98, 070201 (2007).
speaker Dr. Rina Kanamoto (Univ. Arizona)
title Tailor-made condensates using lasers or fermions
abstract I will present the following topics of theoretical studies on cold atoms/molecules:
i) Laser fields with a specific geometry are used to induce a rich varaiety of quantum dynamics of matter wave. I will addres the collapse and revival dynamics of two condensates in a Raman configulation irradiated by helical beams, aiming at testing of the condensate U(1) symmetry.
ii) Polar condensates are shown to be stabilized significantly in an admixture of fermions against collapsing. This fermionic stabilization leads to an emergence of a stable density-wave ground state.

2007/10/2(Tue) 13:00   (at S5 #411)
speaker Hiroaki Terashima (PD)
title Relativistic Effects on the Einstein-Podolsky-Rosen Correlation
abstract The non-local correlation of spins is now widely recognized as an important resource for quantum information processing. We address the issue of how the quantum correlation is modified when seen from observers in motion or in gravity, based on relativistic quantum theory. We then show an apparent decrease in the Einstein-Podolsky-Rosen correlation and discuss the proper way to utilize the perfect correlation.

2007/5/14(Mon) 13:00   (at S5 #411)
speaker Dr. Taeko Matsuura (ECT, Iatly)
title Exact renormalization group approach to the BCS-BEC problem
abstract Recent experimental development of the cold alkali atom gas revealed the BCS-BEC crossover phenomena of the fermi gas with attractive interaction. Theoretically, however, it is still difficult to treat the whole BCS-BEC regime analytically since it contains the strong coupling regime. Exact renormalization group technique is one of the nonperturbative approach which works systematically well even without the small expansion parameter and for whole temperature/density region. In this talk, I describe the renormalization flow of the effective potential and the gap parameter at zero temperature using the one-channel model.

2007/4/11(Wed) 13:30   (at S5 #411)
speaker Dr. Miguel A. Cazalilla (Donostia International Physics Center, Spain)
title Competition between vortex unbinding and tunneling in one-dimensional optical lattices
abstract We study a system of two-dimensional Bose gases trapped in minima of a deep one-dimensional optical lattice potential. Increasing the tunneling amplitude between adjacent gases drives a deconfinement transition from a normal Bose gas where coherence is destroyed by the vortex unbinding phenomenon first described by Berezinskii and Kosterlitz and Thouless, to a phase where coherence is established between neighboring two- dimensional gases. We compute the signature of this transition in the interference pattern of the system as well as in its rotational response, which provides a direct measurement of the superfluidity in the system.