Department of Physics, The University of Tokyo
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2005/2/22 (Tue)
Lecturer Neil Mochan (M1)
Title Spin-2 dynamics in BEC under varying interaction strength
Abstract Recently, Dr. Saito has studied Bose Einstein condensation trapped in free space by manipulating the Feshbach resonance. I study, by computer simulation, how a variation in the three interaction parameters (which were previously constant) could affect the spin dynamics of a spin-2 condensate.

2005/2/8 (Tue) 13:30-
Lecturer Keiji Murata
Title Bose-Einstein condensation with internal degrees of freedom (II)
Abstract In last semester, I presented MFT of the spin-1 Bose-Einstein Condensates and derived the ground state structures. The phase of special interest is the "Mixed phase" which shows the spontaneous symmetry breaking in the case of ferromagnetic interaction. Therefore it is expected that this system has gapless Goldstone mode(s). Obtaining such modes is one of our goals. I'd like to present the Bogoliubov theory formalization of the spin-1 system and examine it in ferromagnetic and polor phases. If possible, I'll show the results in mixed phase and mention about physical interpretation of such modes.

2005/2/4 (Fri) 13:30-
Lecturer Hiroki Saito (Interactive Research Center of Science, Titech)
Title Self-trapped Bose-Einstein condensates stabilized by oscillating interactions
Abstract A matter-wave bright soliton --- a Bose Einstein condensate (BEC) confined by itself without an external potential --- is known to be stable in 1D, in which quantum kinetic pressure counterbalances an attractive interaction. However, in 2D or higher dimensions, self-trapped BECs are always unstable against collapse or expansion as long as the attractive interaction is constant in time. I will show that the self-trapped BEC in 2D can be stabilized by rapid oscillations of the interaction between attractive and repulsive through the Feshbach resonance. The mechanism of the stabilization is similar to that of an inverted pendulum, in which a bob is stabilized above the vertically oscillating pivot. In 3D space, the stabilization of the self-trapped BEC by oscillating interactions has been elusive, since complex dynamical instabilities and strong collapse arise in the 3D system. I will show that if we take into account the effect of dissipation, the self-trapped BEC can be stabilized in 3D.

Lecturer Hiromi ARAI(M1)
Title Observational study on protein expression system
Abstract Cell-free system is very simplified protein synthesis system which produces pure, amount of protein. Because of its simplisity, we estimate that we can extract characteristic behavior of the system, like expression dependence on codon or amino acid, etc. Moreover, because translation process might affects protein folding, I expect that the result of the analysation might become one of the clues of protein folding. In this seminar, I'd like to introduce about what I am analyzing and results I have obtained so far.

2005/1/25 (Tue)
Lecturer Teppei Sekizawa (D1)
Title Superfluid density of weakly interacting spin-1 bosons
Abstract Superfluid density of weakly interacting spin-1 bosons is derived by using Hohenberg-Martin theory when a system is polar, ferromagnetic and super- fragmented phase. We show that the superfluid density which is different in these phases is also the total density at absolute zero as the case of spinless bosons.

2005/1/18 (Tue) 13:00-
Lecturer Hiroaki Terashima (PD)
Title Nonunitary Quantum Circuit
Abstract A quantum circuit is generalized to a nonunitary one whose constituents are nonunitary gates operated by quantum measurement. It is shown that a specific type of one-qubit nonunitary gates, the controlled-NOT gate, as well as all one-qubit unitary gates constitute a universal set of gates for the nonunitary quantum circuit, without the necessity of introducing ancilla qubits. A reversing measurement scheme is used to improve the probability of successful nonunitary gate operation. A quantum NAND gate and Abrams-Lloyd's nonlinear gate are analyzed as examples. Our nonunitary circuit can be used to reduce the qubit overhead needed to ensure fault-tolerant quantum computation.

2005/1/11 (Tue)
Lecturer Takahiro Ohkuma (B4)
Title Statistical Mechanics of Attractive System
Abstract The recent experiments show us the atoms-Molecles crossover by arranging the scatterring length with magnetic field (Feshbach resonance). In this seminar,I will discuss the general way we calculate the grand partition function by means of LEE & YANG cluster expansion method.(T.D.Lee,C.N.Yang Phys.Rev.113,1165(1959)) My motivation is to apply this method to the attractively interacting system which has some bound states.

Lecturer Dr. Kenichi Kasamatsu (Osaka City Univ.)
Title Pattern formation and quantized vortices in two-component Bose-Einstein condensates
Abstract In the seminor, we talk about pattern formation in two-component Bose-Einstein condensates (BECs), focusing on the dynamics of multiple-domain formation and the structure of quantized vortices in rotating BECs. Cross-phase modulation instability gives rise to the formation of multiple domains that alternate two components, which are consistent with the experimental observation by the Ketterle's group. Multicomponent order parameter allows the excitation of various exotic topological defects, e.g., skyrmions or merons, which have no analogue in a system with a single-component order parameter. We discuss their structure in two-component BECs based on the nonlinear sigma model by introduing the "pseudospin". Rapidly rotating two-component BECs exhibit a rich variety of vortex states such as square lattices and vortex sheets of the skyrmions, which depend on the strength of intercomponent interaction and the rotation frequency.

Lecturer Yasuaki Suzuki (B4)
Title Path Integral Approach to a Rotating-Bose-System
Abstract In first semester, I introduce to the paper, gAtomic Theory of the Transition in Helium h by R.Feynman in 1953.In this paper, Feynman showed that liquid He exhibit a transition, using path integral approach. I apply this approach to non-interaction system. In addition, applying to a rotating-trapped-system, I get critical point, which depends on angular-velocity.

2004/11/9 POSTPONED UNTIL 11/16!!!
Lecturer Neil Mochan (M1)
Title Spin Dynamics in Spatially Uniform Bose-Condensed Rb-87
Abstract T. Kuwamoto et al at Gakushuin university recently studied the spin-dependent collision dynamics of Rb-87 spin-2 Bose Einstein Condensates confined in an optical trap and came up with some very interesting results. I make preliminary attempts to simulate their results theoretically, taking for now, the case where the condensate is uniform. I derive the equations of motion for each component and study the time evolution, with a view to eventually being able to fully simulate their observations.

Lecturer Keiji Murata (M1)
Title Bose-Einstein condensation with internal degrees of freedom
Abstract In MIT, Stenger group realized linear form 'spinor BEC' in magnetic field (1998). Linear and quadratic Zeeman terms which are changed arbitrarily play essential role in this system and give us a number of ground state structures by balancing them and interaction energy. Concerning with this, we can draw the phase diagrams of this system and also verify them experimentally. I'd like to show how to obtain them and mention the Bogoliubov spectrums especially the Goldstone modes followed by the spontaneous symmetry breaking in the 'mixed state' in antiferromagnetic case.

Lecturer Hiromi ARAI (M1)
Title Introduction of bioinformatics
Abstract Bioinformatics is a marriage of computer science with molecular biology. Recently data of molecular biology, DNA or protain sequence data etc., have been increasing exponentially. In order to shift out the core information, it is necessary to use informatical method. Moreover, information skill can be used to manage complex science data. In this seminar, i'd like to introduce basis of molecular biology and topics of bioinformatics.

Lecturer Dr. Takashi Mukaiyama (Univ. of Tokyo)
Title Creation of the optically trapped ultracold sodium molecules via a Feshbach resonance
Abstract Recently it has become possible to create ultracold molecular gases from precooled atomic condensates. Feshbach resonance, which originates from the degeneracy of free atomic state and molecular bound state, has been utilized to form molecules in an atomic condensate by adiabatic magnetic-field sweep. Because of the high conversion efficiency and the tunability of the inter-molecular interaction, this method is one of the most promising way to realize momentum-correlated pair of atoms (BCS pair), and to study BEC-BCS cross over of paired atoms.
Here we present the experimental result on the formation of sodium molecules in an atomic bose condensate. We have produced 10^5 molecules, which corresponds to the phase space density of more than 20 in our trap conditions. We have measured the typical evidence of the condensation of "falling cloud like a rock." We have also studied the dissociation and decay of those molecules in the negative scattering-length side of resonance, where BCS paired state exists for molecules composed of fermions. Understanding the dissociation mechanism is crucial for the discussion of what the smoking gun of BCS pairing is.

Lecturer Dr. Yuki Kawaguchi (Kyoto Univ.)
Title Vortex state in a superfluid Fermi gas near a Feshbach resonance
Abstract I discuss the vortex state in the BCS-BEC crossover region. Using a Feshbach resonance, it is possible to study the crossover between a BCS superfluid of Fermi atoms and a Bose Einstein condensate (BEC) of molecules. In a pure molecular condensate at zero temperature, there is no normalfluid in a vortex core. On the other hand, far from the resonance in BCS side, a vortex core is filled with atoms. The microscopic properties of a single vortex in the crossover region are studied by using self-consistent Bogoliubov-de Gennes theory. Crossing a Feshbach resonance from BCS to BEC side, the number of atoms decreases and, moreover, the fraction of atoms in the vortex core decreases because of the existence of the molecular BEC.

Lecturer Teppei Sekizawa (D1)
Title Superfluid Density of Weakly Interacting Bosons on a Surface of a Rotating Torus
Abstract We use linear response theory generalized to fit a rotating system to show that the superfluid density of interacting bosons on the surface of a rotating torus becomes less than the total density even at 0 K. This reduction of the superfluid density is shown to originate from the correlation of non-condensate bosons. Our prediction can be tested by measuring the superfluid density of liquid $^4$He in a rotating porous media with torsional pendulum. llpadding="5">

Lecturer Dr. Tetsufumi Tanamoto (Toshiba R&D Center)
Title Charged qubit based on coupled quantum dots and measurement process
Abstract I theoretically discuss charged qubits. The outline is following:
(1) Charged qubit Hamiltonian and its similarity to NMR Hamiltonian --- I derive charged qubit Hamiltonian from capacitance network of quantum dots.
(2) Charged qubit in Si-MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor) --- By using conventional MOSFET current formula, I consider the possible number of qubits that can be readout.
(3) Measurement process by single electron transistor(SET) and quantum point contact(QPC) --- By using density matrix equations, I describe the time-dependent behavior of detector current of SET and QPC.

Lecturer Prof.Anthony J. Leggett(University of Illinois)
Title BEC (Bose-Einstein Condensation): a "tapas" of topic
Abstract I discuss the following topics related to Bose-Einstein condensation in the alkali gases:
(1) What is BEC?
(2) The relation between the Gross-Pitaevskii and Bogoliubov descriptions.
(3) What is superfluidity?
(4) Limits on the condensate fraction.
(5) Detecting the normal component in Ramsey-fringe type experiments.