UEDA GROUP

Department of Physics, The University of Tokyo

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2002年度

**2003/2/18(Tue) 16:00から**

Lecturer | 関澤鉄兵(M1) |
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Title | A derivation of the Chern-simons action |

Abstract | We present a systematic derivaton of the Chern-simons action for the Fractional Quantum Hall Effect. |

**2003/2/10(Mon) 12:30から**

Lecturer | 梅澤岳央(M1) |
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Title | Squeezed Spin State at Non-zero temperature |

Abstract | It is doubtful that we achieve to make a pure state, so we add the thermal disturbance to get a mixed state model, and examine how the system evolves under the influence of One-Axis twisting operator. It led some thermal threshold for achieving a squeezed spin state. |

**2003/2/6(Thu) 13:30から**

Lecturer | 森隆博(M2) |
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Title | Bogoliubov theory of spin-1 BECs with linear and quadratic Zeeman effect in traps |

Abstract | In previous my seminar, I discussed Bogoliubov theory of spin-1 BECs in the case of spatialy uniform. However, if we want to predict this system for experiment, we should consider traps. In this seminar, I would like to discuss Bogoliubov theory of spin-1 BECs in traps. |

**2003/1/27(Mon) 13:00から**

Lecturer | 藤井友晃(M2) |
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Title | Properties of weakly interacting boson systems under rotation |

Abstract | Weakly interacting boson systems have some exact solutions by using Lowest Landau Level approximation. In this seminar we present these solutions and their properties. |

**2003/1/20(Mon) 13:00から**

Lecturer | 金本理奈(D2) |
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Title | Rotational properties of one-dimensional Bose gases with attractive interactions |

Abstract | The superfluid properties of attractive BEC in a rotating torus are discussed. Under the adiabatic control of parameters (strength of interaction and angular frequency of the torus), the plane wave state survives below the critical strength of interaction; otherwise the ground state is soliton in the mean-field theory. By studying excitation from each ground state in the Bogoliubov theory and beyond, we show the robustness of the quantized circulation. |

**2002/12/2(Mon) ※月曜３：００から理学部小会議室にて**

Lecturer | 古沢明 氏 (東大工) |
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Title | 量子エンタングルメント工学 |

Abstract |
近年技術の進歩により、過去に思考実験として考えられていた事柄が、実験室で実際
に実験可能となってきている。また、ムーアの法則に代表されるような情報処理素子
の微細化により、量子力学的法則に支配される量子の世界が目の前に迫っている。つ
まり、LSIはそのスケールが量子力学的領域に踏み込もうとしており、ニュートン力
学では説明できない「重ね合わせの状態」や「量子エンタングルメント」などの「怪
現象」が、誤動作を引き起こしかねない状況になりつつある。この状況をネガティブ
に捉えず、「怪現象」を積極的に利用していこうというのが量子情報科学であり、現
在多くの場所で研究が行われている。 本講演では、このような「怪現象」の本質である量子エンタングルメントとその応 用（量子エンタングルメント工学）について、量子光学実験の立場から述べる。 |

**2002/11/26(Tue)**

Lecturer | 松葉一顕(B4) |
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Title | Quantum circuit (basic of quantum computation) |

Abstract | Quantum computer can solve some problems that it is hard to solve by using classical computer. I explain some fundamental quantum circuits which constitute a quantum computer. |

**2002/11/5(Tue)**

Lecturer | 藤井芳孝(B4) |
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Title | Non-Unitary Quantum Computing |

Abstract | "A quantum system's time development is described as an unitary operator." By such common knowledge, we considered only unitary quantum computing until now. On the other hand, we know observations are not unitary. I want to talk about new quantum computer by observations. |

**2002/10/28(Mon) ※月曜１３：００から**

Lecturer | 大橋洋士 氏 (筑波大) |
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Title | BCS-BEC Crossover in a Trapped Gas of Fermi Atoms with a Feshbach Resonance |

Abstract |
We present a theoretical study of the BCS-BEC (Bose-Einstein
condensation) crossover in a trapped gas of Fermi atoms including the
effect of a Feshbach resonance. Going past the simple mean field
approximation, we include the effect of fluctuations in the strong
coupling limit involving two kinds of Bose molecules, i.e., Feshbach
molecules associated with the resonance and preformed Cooper-pairs in
which the pairing interaction is mediated by Feshbach
quasi-molecules. The superfluid phase transition (Tc) is shown to change
continuously from the BCS-type to a BEC of these two kinds of Bosons,
as the threshold energy of the Feshbach resonance is lowered. This BCS-BEC crossover phenomenon strongly affects the atomic density profile at Tc, reflecting the change of the dominant particles going from Fermi atoms to composite Bosons (Feshbach molecules and preformed Cooper-pairs). In addition, in the BEC regime, these composite Bosons are shown to first appear well above Tc. We introduce a characteristic temperature describing the effective crossover from a Fermi gas to a gas of stable molecules in the normal phase. We also discuss the BCS-BEC crossover in the superfluid phase below Tc. In particular, we will discuss how the collective modes vary as we pass through the crossover region. 参考文献： Y. Ohashi and A. Griffin: Phys. Rev. Lett. 89 (2002) 130402. |

**2002/10/22(Tue)**

Lecturer | 梅澤岳央(M1) |
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Title | About a squeezed spin state |

Abstract | A squeezing in the spin system has been studied since 1970's. We might not say the history of this subject is fully developed, thus it is not so easy to understand for beginners like me. In this seminar, I'd like to start from the basic quantum optics. Then I will discuss the squeezed spin state using analogy with the bose systems discussed in quantum optics. At last I'd like to explore to discuss the spin squeezing of mixed systems. |

**2002/10/15(Tue) ※１７：３０から**

Lecturer | 藤井友晃(M2) |
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Title | Analytical method for weakly interacting Bose-Einstein codensates under rotation |

Abstract | We consider a system of weakly interacting N bosons in a two dimensional harmonic traps with total angular momentum L. In this system, by using lowest-Landau level approximation. we can obtain some explicit analytical results of eigenstates of L .These obtained eigenfunctions are expressed as simple symmetric polynomials which contains yrast state . |

**2002/10/8(Tue)**

Lecturer | 吉川豊 氏 (東大総合文化) |
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Title | 非共鳴光下におけるボース凝縮気体の電歪効果 |

Abstract | 電歪とは電場によって引き起こされる誘電体の密度変形の一形態で、気体に対しては 通常無視される。しかし空間的に非常に大きな広がりを持つボース凝縮気体において はこの効果が顕著に現れ、均一な平面波を照射するだけで密度分布が変形を起こす。 講演では理論的アプローチや実験的に観測されたボース凝縮体の密度分布の変化の様 子を紹介する。 |

**2002/10/3(Thu) ※木曜日１３：００から**

Lecturer | 関澤鉄兵(M1) |
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Title | Flat-band ferromagnetism in Hubbard Models |

Abstract | We treat a new class of flat-band models which have two or more internal sites. We show that our new models exhibit ferromagnetism in various numbers of electrons by changing parameters. |

**2002/9/26(Thu) ※木曜日１３：００から**

Lecturer | 森隆博(M2) |
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Title | Bogoliubov theory of spin-1 Bose-Einstein condensates with the quadratic Zeeman effect |

Abstract | We solved analytically the ground state and the excitation spectrum of spin-1 Bose-Einstein condensates(BECs) with the linear and quadratic Zeeman effect by using mean-field theory. In these results, we discovered the phase which has never been obtained by considering only the linear Zeeman effect. In this phase, there is new spontaneous symmetry braking. We argue therefore that this is new phase. We also show new excitation spectra which are ``clockwise'' and ``counter clockwise'' spin fluctuations in ``polar'' phase. |

**2002/9/17(Tue)**

Lecturer | 金本理奈(D2) |
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Title | Quantum phase transition of One-dimensional Bose-Einstein condensates with attractive interactions |

Abstract | Motivated by the recent development of the Feshbach technique, I will discuss the ground and low-lying excited states of attractive Bose-Einstein condensate on a one-dimensional ring as a function of the strength of interactions. The sharp second-order quantum phase transition between the uniform and bright-soliton states appears in the mean-field theory, and the lowest excitation spectrum exhibits a singular cusp at the critical point in the Bogoliubov theroy. However, the exact diagonalization of the many-body Hamiltonian removes the singularity of the transition and changes the properties of the two-body correlations and condensate fraction due to the quantum fluctuations. |