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低温玻色系统的量子动力学研究
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摘要
1924年印度科学家S. N.玻色完成一篇关于光子气体的统计性质的文章,接着在第二年A.爱因斯坦指出在一定临界温度下,无相互作用的原子可以在基态存在宏观的凝聚。在之后的70多年中,玻色-爱因斯坦凝聚成为物理学中具有重大吸引力的课题之一,许多伟大的物理学家,包括A.爱因斯坦,伦敦兄弟,L.D.朗道,N. N.波戈留波夫,R. P.费曼……,都在这方面做出过贡献。实验的突破发生在1995年,美国科学家在碱金属的稀薄原子气体中实现了几乎纯净的玻色-爱因斯坦凝聚体;通过调节外囚禁势的频率,一维、二维凝聚体跟着制备出来,其它一些重要的进展包括宏观凝聚体的相干现象,在光学格子势中实现超流-绝缘体相变,在玻色凝聚体中制备孤子以及通过Feshbach技术调节原子之间的相互作用强度等等。理论方面的突破始于宏观波函数概念的提出,它极大的推动了玻色-爱因斯坦凝聚理论的发展,并且成为分析凝聚体性质的重要方法。然而即使在接近零温的凝聚体中,原子之间的相互作用引起的量子波动也是一个不可忽略的现象,是分析凝聚体的稳态和动力学行为的一个重要因素。本文从理论的角度出发,着重分析超低温有限粒子数目玻色系统的多体动力学,内容主要集中在一维吸引相互作用凝聚体的多体动力学和双阱势中两分量凝聚体的纠缠现象上。
     1.在一维环状势中的玻色凝聚体,如果原子之间是吸引的相互作用,平均场理论预测在较弱的相互作用强度时系统的基态是均匀分布的状态,当相互作用强度超过特定临界值后,基态局域化为亮孤子状态。两个状态对应的化学势连续但是一阶导数不连续,存在量子相变。在系统是亮孤子状态时,原子之间的相互作用强度较大,量子波动必然具有明显的影响。基于这样的考虑,我们从多体动力学的角度,通过计算系统的仙农(Shannon)熵、动量分布等物理量,分析了量子波动对系统的动力学演化在不同参数区间的影响。我们的计算结果不仅是对平均场理论的补充,同时表明在强相互作用下,高角动量态显著的激发,平均场理论不能全面描述系统的特性。
     2.分析了双阱势中两分量凝聚体的能级和纠缠特性。从两模近似出发,我们首先计算了系统的能谱。发现在增加同种原子和不同种原子之间的相互作用强度时,能谱存在明显的反交叉。接着分析了系统的动力学演化,如果初态选择两种分量分别处在两个不同的势阱中,已有的理论分析显示在弱和强相互作用极限下,原子在势垒间的隧穿是以不同原子之间耦合成对的形式进行。我们的计算结果表明,当系统的对隧穿形式占主要的时候,二阶交叉关联函数违背Cauthy-Schwarz不等式,纠缠的程度可以用Von-Neumann熵度量。
In 1924, S. N. Bose wrote a paper on the statistics of photons. Following thiswork, A. Einstein considered a gas of non-interacting atoms and concluded thata macroscopic fraction of the total atoms will occupy the lowest-energy single-particle state below a certain critical temperature. In the following seventy years,Bose-Einstein condensation becomes one of the most exciting fields in modernphysics. Many famous physicists have contributed to this field. The list of thenames includes, for example, A. Einstein, F. London and his brother H. Lon-don, L. D. Landau, N. N. Bogoliubov, R. P. Feynman... ... The most importantadvancement of experiment happened in 1995. Three individual groups in USAgenerated almost purified Bose condensates of dilute alkalis gases in harmonictraps. By adjusting the frequencies of external traps, one- and two-dimensionBose condensates are generated too. Other important and interest experimen-tal results include the interference between two independent condensates, thesuper?uid-insulator transition of Bose gas in optical lattice, dark and brightsolitons in the condensates and tuning the interaction strength of atoms withFeshbach resonance,and so on. The breakthrough on the theory of Bose-Einsteincondensation started from the combination of condensates and macroscopic wavefunction. It is convenient and promotes the development of theory of condensates.Nowadays, it becomes one of the important tools on analyzing the condensates.On the other hand, quantum correlation e?ects plays an unique role in the studyof condensates, which in?uences the ground state and dynamics of the systemeven at almost zero temperature. In this thesis, we study the many-body dy-namics of finite Bose gas at low temperature. The work concentrates on thequantum dynamics of one dimensional Bose gas with attractive interaction andthe entanglement of two-component Bose condensates in a double-well.
     1. We investigate the many-body dynamics of an e?ectively attractive one-dimensional Bose system confined in a toroidal trap. The mean-field theorypredicts that a bright-soliton state will be formed when the interparticle inter- action increases over a critical point. The chemical potential is continuous whilethe first order derivation of the chemical potential is discontinuous. Thus thereis a quantum phase transition at the critical point. Due to the strong interac-tion strength of atoms in a soliton, quantum correlation has to be considered.The study of quantum many-body dynamics in this thesis reveals that thereis a modulation instability in a finite Bose system correspondingly. We showthat Shannon entropy becomes irregular near and above the critical point due toquantum correlations. We also study the dynamical behavior of the instabilityby exploring the momentum distribution and the fringe visibility, which can beverified experimentally by releasing the trap.
     2. We consider a novel system of two-component atomic Bose–Einsteincondensate in a double-well potential. Based on the well-known two-mode ap-proximation, we demonstrate that there are obvious avoided level crossings whenboth interspecies and intraspecies interactions of two species are increased. Thequantum dynamics of the system exhibits revised oscillating behaviors comparedwith a single component condensate. We also examine the entanglement of twospecies. Our numerical calculations show that the onset of paired-tunneling canbe signalled by a violation of the Cauchy–Schwarz inequality of a second-ordercross-correlation function. Consequently, we use Von Neumann entropy to quan-tify the degree of entanglement.
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