Department of Physics, The University of Tokyo
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2006/1/26 (Thu) 16:00

Lecturer Neil Mochan (M2)
Title Spin Dynamics in an optically trapped, spatially unifrom F=1 Bose Einstein Condensate
Abstract Optical trapping of a BEC removes the tendency for the atoms to exist in a single hyperfine magnetic state. This new degree of freedom allows spin dynamics to occur. For a spatially uniform condensate in the absence of a magnetic field, the dynamics between the m=1,0,-1 states is modelled over varying initial conditions and compared to recent experimental data. Also studied is the situation where the interaction is oscillated by varying the s-wave scattering lengths by sweeping over the Feshbach resonance.

2006/1/17 (Tue)

Lecturer Yuki Kawaguchi (PD)
Title Einstein-de Haas effect in dipolar Bose-Einstein condensates
Abstract The magnetic dipole-dipole interaction couples the spin and orbital angular momenta so that spin relaxation causes the system to rotate mechanically (Einstein-de Haas effect) or, conversely, a solid-body rotation of the system leads to its magnetization (Barnett effect). We show that these effects also occur in a dipolar Bose-Einstein condensate in which atoms undergo scalar, spinor, and dipolar interactions. General properties of the order parameter for a dipolar spinor Bose-Einstein condensate is discussed based on symmetries of the interactions, and an initially spin-polarized dipolar condensate is shown to dynamically generate a new type of non-singular vortex via spin-orbit interactions. We also discuss the effect of the external magnetic field and that of the trap geometry on the properties of the condensate.

2005/12/6 (Tue)

Lecturer Shuta Nakajima (B4)
Title Quantum phase transition from a superfluid to a Mott insulator in a gas of ultracold atoms
Abstract In 2002, I. Bloch et al. observed a superfluid Mott-insulator phase transition in a Bose-Einstein condensate held in a three-dimensional optical lattice potential. As the potential depth of the lattice is increased, a transition is observed from a superfluid to a Mott insulator phase. In the superfluid phase, each atom is spread out over the entire lattice, with long-range phase coherence. But in the insulating phase, exact numbers of atoms are localized at individual lattice sites, with no phase coherence across the lattice. In this seminar, I provide brief explanations about optical lattice, Bose-Hubbard model and superfluid Mott-insulator transition. And then, I introduce the experiment fulfilled by Bloch et al.

2005/11/29 (Tue)

Lecturer Tomoya Ono (B4)
Title Coherent collisional spin dynamics in optical lattices
Abstract Immanuel Bloch and his collaborators observed coherent, purely collisionally driven spin dynamics of rubidium87 atoms in an optical lattice. For high lattice depths, atom pairs confined to the same latteice site show weakly damped Rabi-type oscillations between two-particle Zeeman states of equal magnetization, induced by spin changing collisions. I perform a review about their paper.

2005/11/22 (Tue)

Lecturer Yuji Kurotani (M1)
Title Theoretical circuit analysis of quantum measurement processes
Abstract Measurement is to obtain information about a measured system. von Neumann have proposed that we can decompose any quantum measurement processes into two parts. The first part is a unitary transformation to move the information from the measured system to the measuring apparatus. The second part is the projective measurement on the apparatus (non-unitary transformation) to obtain the information about the initial system state. If we obtain the information of the system precisely, it seems that the post-measurement state of the system must change into an eigenstate of the measured observable. However, it is not universally true when using the `contractive state measurement (CSM)' proposed in 1980's. The CSM is an epoch-making model, but there is a little problem. In this seminar, I explain `swapping state measurement' as a improvement model of the CSM by using quantum circuit. These are usually discussed in continuous variable system, but we can also discuss them in spin-1/2 system.

2005/11/15 (Tue)

Lecturer Dr. David Roberts (Ecole Normale Superieur)
Title Casimir-like drag in a slow-moving Bose-Einstein condensate
Abstract It is widely accepted that a superfluid flow exhibits a critical velocity below which there is no dissipation. However, the often-neglected zero-temperature quantum fluctuations have implications for the existence of this critical velocity. The drag force on an object created by the scattering of these quantum fluctuations in a three-dimensional, weakly interacting Bose-Einstein condensate is discussed. A non-zero force at low velocities is found to exist for two specific experimentally realizable examples, which suggests that the effective critical velocity in these systems is zero.

2005/11/8 (Tue)

Lecturer Hiromi Arai (M2)
Title Protein stability analysis by statistical learning
Abstract Proteins are by far most structurally complex and functionally sophisticated molecules known. A protein molecule is made from a long chain of amino acids. There are 20 types of amino acids. Each type of protein has a unique sequence of them and, surprisingly, has a unique fold: the protein chain FOLDS in its unique stable form and exists in the physiologic condition. The mechanism of protein folding is still remain unsolved and prediction of protein fold by its sequence is left unfulfilled because of complexity of the system. Moreover, not all protein folds: folding condition is sensitive and limited sequence folds stable. But recently statsitical learning method has been developed and performs modestly. In my study, I marked on the critical point whether one protein is stable or not amd have been made a statistically analysis. In this seminar I am going to give a brief review of bioinformatics around my study and introduce what I am studying.

2005/10/25 (Tue)

Lecturer Keiji Murata (M2)
Title Bogoliubov excitations on spontaneously-broken axisymmetry phase of f=1 spinor BEC
Abstract Spinor BEC system in the magnetic field is interesting for the rich ground state structures depending on the linear and quadratic Zeeman effects. MFT approach shows the characteristic phase of the f=1 spinor BEC, called `mixed' phase in MIT paper, is the phase that has spontaneously-broken axisymmetry. That is, the magnetization in this phase is not parallel to the applied magnetic field but tilted against it. We formulated the Bogoliubov theory on spinor BEC and verified the existence of gapless Goldstone modes predicted by the Goldstone's theorem. The interpretations of them are also very characteristic because they are phonon-magnon coupled excitation modes (In other words, they are such modes that recover the spontaneously-broken U(1) and SO(2) symmetries "simultaneously"). In this seminar, I will explain how to get this conclusion analytically and display the numerical verification of them.

2005/10/18 (Tue)

Lecturer Dr. Michikazu Kobayashi (Osaka City Univ.)
Title Localization of Bose-Einstein Condensation and Disappearance of Superfluidity of Strongly Correlated Bose Fluid in a Confined Potential
Abstract Recently, some experiments of liquid 4He confined in porous glass have found the signal of Bose-Einstein condensation without superfluidity, suggesting a new phase transition to localized BECs (Bose glass). Motivated by the recent observation of quantum phase transition at high pressures and near the zero temperatures, observed by Yamamoto et al., we develop the three-dimensional Bose fluid in a confined potential. By introducing the localization length of localized condensates, we make a new analytical criterion for the localization of the Bose condensate. The critical pressure of the transition from normal condensate to localized condensates is quantitatively consistent with observations without free parameters.

2005/10/11 (Tue)

Lecturer Teppei Sekizawa (D2)
Title Quantum Statistical Machanics of Spin-1 Hard-Sphere Bose Gas with Spin-dependent Interaction
Abstract We discuss statistical mechanics of spin-1 hard-sphere Bose gas with spin-dependent interaction by using Lee-Yang work in 1959. Although they studied spin-1 hard-sphere Bose gas with spin-independent interaction, we first study spin-1 hard-sphere Bose gas with spin-dependent interaction by using Lee-Yang method. We discover the binary kernel in our system and calculate the pertition function when the system does not exhibit Bose-condensation.

2005/9/6 (Tue) 13:30

Lecturer Dr. Miguel A. Cazalilla (Donostia International Physics Center)
Title "Breaking up" the BEC
Abstract In recent years, interest in strongly correlated systems is driving us beyond the study of phenomena related to Bose-Einstein condensates (BEC) in weakly interacting gases of ultracold atoms. In this seminar, we shall focus on two much investigated routes to "break up" the BEC: low-dimensional systems in optical lattices and fast rotating Bose gases. Whereas the former has already been realized in various kinds of experiments, the latter still remains an exciting playground for theorists (although experiments are getting closer and closer to the strongly correlated regime). In the first part of the seminar, we shall discuss our recent results on the phase diagram and excitation spectra of two-dimensional (or very anisotropic) optical lattices. These are described as arrays of one-dimensional Bose gases. We compare our results with the experiments of the Zurich group, and show that they are, at least, in good qualitative agreement. In the second part of the seminar, we shall discuss the low-energy excitations of rapidly rotating Bose gases. We consider the so-call quantum Hall regime, where this system exhibits a series of vortex liquid states and it is no longer a BEC. We use exact numerical diagonalization of small atom systems to study the excitation of the vortex liquids in a harmonic trap. The low-lying excited states turn out to be surface (edge) waves. Comparison with existing theories for the edge excitations of quantum Hall states will be also presented. We thus find that the Pfaffian (or Moore-Read) state presents a number of anomalies, which may have their origin in a reconstruction of the edge of this vortex liquid.

2005/7/1 (Fri) 15:00-17:00 理学部第二会議室 (H3-45)

Lecturer Prof. Sandro Stringari (Trento Univ.)
Title Bloch oscillations in ultra-cold atoms and test of the Casimir-Polder force
Abstract After a brief summary of recent theoretical and experimental work on Bloch oscillations in ultracold gases, I will discuss possible applications to the inteferometic study of the Casimir-Polder force generated by the surface of a substrate on a single atom. Recent work on the surface-atom force at the micron distance will be reviewed and new perspectives opened by atomic physics in this area will be outlined.
詳細 http://www.phys.titech.ac.jp/coe21/seminar/index.html