超冷铷原子气体量子相变的研究

发布时间：2018-04-13 04:24

本文选题：玻色-爱因斯坦凝聚体 + 一维光晶格　；参考：《中国科学院研究生院(武汉物理与数学研究所)》2016年博士论文

【摘要】：上个世纪末,稀薄气体中玻色-爱因斯坦凝聚体(BEC)的实验实现引起了量子简并气体研究的热潮。而光学晶格因其晶格常数的可调节性和光学阱深易控制的特点,也被广泛地应用于超冷原子实验的研究。光晶格和BEC的结合,可以为我们展示非常丰富的量子现象。本论文研究了一维光晶格中BEC的相位涨落和超冷玻色量子临界气体密度概率分布的普适规律,以及静磁阱中超冷玻色气体在临界区域内的动量分布和BEC相变温度以上的一阶空间相干性。主要的工作成果体现在以下几个方面：1.提出并从实验上验证了一种测量光晶格中BEC相位涨落的新方法。该方法是从光晶格中释放的原子气体密度分布的傅里叶功率谱中提取相位涨落。与基于干涉图样的对比度中提取相位涨落的传统方法相比,我们的方法在光晶格的阱深较深,干涉峰无法辨识的情况下依然适用。2.研究了一维光晶格系统中超冷玻色量子临界气体密度概率分布的普适规律。发现当系统进入了Berezinskii-Kosterlitz-Thouless(BKT)相变以上的量子临界区域时,从光晶格中释放的原子气体的密度概率服从指数分布。当同时考虑晶格格点之间以及格点内部原子间的相位涨落,我们给出的理论模型可以很好的解释这种普适规律。3.研究了静磁阱中BEC相变温度附近的超冷玻色气体的动量分布。通过对空间上处于临界区域内原子气体的动量分布半高宽进行统计,发现原子气体的温度非常接近BEC相变温度Tc时,动量分布的半高宽突然减小,表现出十分明显的奇点行为。临界气体的动量分布的观测为我们理解临界行为提供了一个新的视角。4.采用Kapitza-Dirac散射的办法,研究了静磁阱中BEC相变温度以上超冷玻色原子气体的一阶空间相干性。通过对干涉图样对比度的统计,发现温度非常接近相变温度时,原子气体的相干性明显好于热原子；随着温度的升高,原子气体的相干性逐渐减弱,最终与热原子的相干性完全相同。
[Abstract]:At the end of last century, the experimental realization of Bose-Einstein condensate (BECs) in rarefied gases has caused a boom in the study of quantum degenerate gases.The optical lattice is widely used in the study of ultra-cold atomic experiments because of its adjustable lattice constant and easy control of the depth of the optical well.The combination of optical lattice and BEC can show us a very rich quantum phenomenon.In this paper, the phase fluctuations of BEC and the probability distribution of the quantum critical gas density of supercooled boson in one-dimensional optical lattice are studied.The momentum distribution in the critical region and the first-order spatial coherence above the BEC phase transition temperature of the ultra-cooled Bose gas in the magnetostatic trap are also discussed.The main achievements of the work are as follows: 1.A new method for measuring BEC phase fluctuations in optical lattices is proposed and experimentally verified.The method is to extract the phase fluctuation from the Fourier power spectrum of the atomic gas density distribution released in the optical lattice.Compared with the traditional method of extracting phase fluctuation from contrast based on interference pattern, our method is still applicable in the case of deep well depth of optical lattice and unidentifiable interference peak.The probability distribution of supercooled boson quantum critical gas density in one-dimensional optical lattice system is studied.It is found that when the system enters the quantum critical region above the Berezinskii-Kosterlitz-Thoulessitz-BKT phase transition, the density probability of the atomic gas released from the optical lattice is exponentially distributed.When the phase fluctuations between lattice points and atoms within lattice points are considered at the same time, we give a theoretical model that can explain this universal rule.The momentum distribution of supercooled Bose gas near the BEC phase transition temperature in a magnetostatic trap is studied.By statistical analysis of the half-maximum width of the momentum distribution of the atomic gas in the critical region, it is found that when the temperature of the atomic gas is very close to the BEC transition temperature T _ c, the half-maximum width of the momentum distribution suddenly decreases, showing a very obvious singularity behavior.The observation of the momentum distribution of the critical gas provides us with a new angle of view to understand the critical behavior.The first order spatial coherence of supercooled Bose gas above the BEC phase transition temperature in a magnetostatic trap is studied by Kapitza-Dirac scattering method.Through the statistics of the contrast of the interference pattern, it is found that the coherence of the atomic gas is obviously better than that of the thermal atom when the temperature is very close to the phase transition temperature, and the coherence of the atomic gas decreases with the increase of the temperature.In the end, the coherence of the thermal atoms is exactly the same.
【学位授予单位】：中国科学院研究生院(武汉物理与数学研究所)
【学位级别】：博士
【学位授予年份】：2016
【分类号】：O469

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