[论文解读] Correlations and dynamics of tunnel-coupled one-dimensional Bose gases

Thomas Schweigler|arXiv (Cornell University)|Aug 1, 2019

Cold Atom Physics and Bose-Einstein Condensates参考文献 75被引用 23

一句话总结

本论文通过物质波干涉测量空间相位关联，实验研究了双势阱中隧穿耦合的一维超冷玻色气体。通过在初始隧穿后突然断开双阱耦合，首次观测到‘高斯化’现象——即相位涨落在动力学演化中从非高斯态转变为高斯态，揭示了量子多体系统通过幺正演化弛豫至高斯平衡态的机制。

ABSTRACT

We present a series of experiments performed with two ultracold one-dimensional Bose gases (rubidium atoms) in a double well potential. Employing matter-wave interference, we can measure the spatially resolved phase difference between the two gases and consequently investigate spatial correlations. By investigating whether higher order correlation functions can be factorized into correlations of lower order, we can investigate the interaction properties of the system. For a non-interacting system, all correlation functions with orders greater than two factorize and one observes Gaussian fluctuations. Here, we present the measurement of non-factorizing fourth-order correlation functions, leading to an experimental characterization of the interactions between the collective excitations of the quantum many-body system. The degree of non-factorizibility, i.e., the degree of non-Gaussianity of the phase fluctuations, depends on the tuneable tunneling strength between the wells. Starting from such a non-Gaussian state, we are able to observe the dynamical evolution towards a state with factorizing correlation functions (Gaussian fluctuations). We start in a double well with tunneling and then abruptly decouple the two subsystems. Subsequently, we observe how the initially non-Gaussian phase fluctuations become Gaussian. Moreover, we discuss the dynamical emergence of phase coherence in a double well potential with tunneling. We experimentally investigate the evolution starting from two different initial states. In one case, we split a cloud of atoms into two and trigger global oscillations in their relative phase. The oscillations subsequently damp and phase coherence sets in. In the other case, two independent clouds are suddenly coupled by tunneling. Again, phase coherence emerges between the two subsystems.

研究动机与目标

• 理解量子多体系统的动力学，特别是高斯平衡态的出现机制。
• 实验探测超冷一维玻色气体中的非高斯关联。
• 研究在不同初始条件下，隧穿耦合系统中相位相干性的形成过程。
• 检验关于相互作用量子系统中高阶关联函数可分解性的理论预测。
• 表征隧穿强度在决定相位涨落非高斯性程度中的作用。

提出的方法

• 将超冷铷原子装载至双势阱中，形成两个隧穿耦合的一维玻色气体。
• 通过调节阱间隧穿强度，控制系统的相位相干性与非高斯性。
• 利用物质波干涉技术测量空间分辨的相位差。
• 通过实验测量四阶关联函数，探测不可分解（非高斯）涨落。
• 通过提高阱间势垒高度，突然断开双阱耦合，研究关联函数的动力学演化。
• 制备两种不同初始态：具有相干振荡的分裂原子云，以及通过隧穿突然耦合的两个独立原子云。

实验结果

研究问题

• RQ1 在隧穿耦合的一维玻色气体中，相位涨落的非高斯性程度如何随隧穿速率变化？
• RQ2 仅通过幺正演化，非高斯量子态能否动态演化为高斯态，而无需外部耗散？
• RQ3 集体激发在决定此类系统中高阶关联函数结构方面起什么作用？
• RQ4 当从两个独立子系统出发时，相位相干性在双势阱中如何形成？
• RQ5 哪些实验特征可区分不可分解的四阶关联与可分解的关联？

主要发现

• 本实验首次直接观测到‘高斯化’现象——即在双势阱系统突然断开后，初始非高斯相位涨落的动力学弛豫至高斯态。
• 测量到不可分解的四阶关联函数，证实了系统中存在强多体关联与非高斯统计特性。
• 相位涨落的非高斯性程度可通过隧穿速率直接调控，表明对系统统计性质的可控性。
• 在两种初始条件下，相位相干性均得以形成：一种来自分裂原子云的相干叠加，另一种来自独立子系统通过隧穿的突然耦合。
• 观测到的高斯涨落弛豫过程不依赖外部耗散，表明仅通过幺正量子演化即可使多体系统达到高斯平衡态。
• 结果验证了理论预测：关联函数中的非高斯性表明低维量子气体中存在强相互作用与集体行为。