[论文解读] Observation of metastability in an open quantum system with long-range interactions

Lorenz Hruby, Nishant Dogra+3 more|arXiv (Cornell University)|Aug 7, 2017

Cold Atom Physics and Bose-Einstein Condensates被引用 1

一句话总结

本研究在光学腔耦合的三维超冷 87Rb 气体中观测到非平衡态下的亚稳态，该系统处于光晶格中，长程光子介导的相互作用驱动从 Mott 绝缘体向密度波相变。通过调节全局耦合强度，系统表现出迟滞和类似雪崩的隧穿过程，涉及数千个原子，证实了由于短程与长程相互作用竞争导致的亚稳态行为。

ABSTRACT

We experimentally study the stability of a bosonic Mott-insulator against the formation of a density wave induced by long-range interactions, and characterize the intrinsic dynamics between these two states. The Mott-insulator is created in a quantum degenerate gas of 87-Rubidium atoms, trapped in a three-dimensional optical lattice. The gas is located inside and globally coupled to an optical cavity. This causes interactions of global range, mediated by photons dispersively scattered between a transverse lattice and the cavity. The scattering comes with an atomic density modulation, which is measured by the photon flux leaking from the cavity. We initialize the system in a Mott-insulating state and then rapidly increase the global coupling strength. We observe that the system falls into either of two distinct final states. One is characterized by a low photon flux, signaling a Mott insulator, and the other is characterized by a high photon flux, which we associate with a density wave. Ramping the global coupling slowly, we observe a hysteresis loop between the two states - a further signature of metastability. A comparison with a theoretical model confirms that the metastability originates in the competition between short- and global-range interactions. From the increasing photon flux monitored during the switching process, we find that several thousand atoms tunnel to a neighboring site on the time scale of the single particle dynamics. We argue that a density modulation, initially forming in the compressible surface of the trapped gas, triggers an avalanche tunneling process in the Mott-insulating region.

研究动机与目标

• 研究在长程相互作用存在下，Mott 绝缘体对密度波形成的稳定性。
• 表征开放量子系统中 Mott 绝缘体与密度波相之间的本征动力学行为。
• 确定系统中亚稳态的起源及其与相互作用范围和耦合强度的关系。
• 观测并量化由初始密度调制触发的可压缩表面层中的隧穿动力学。

提出的方法

• 在三维光晶格中制备 87-铷原子的量子简并气体，并通过横向晶格散射与光学腔耦合。
• 光子在原子气体与腔之间发生非简并散射，介导全局范围的长程相互作用。
• 通过从腔中泄漏的光子通量探测原子密度调制，作为系统状态的实时测量。
• 通过快速或缓慢调节全局耦合强度，诱导 Mott 绝缘体与密度波相之间的相变。
• 通过正向与反向的耦合强度扫描测量迟滞回线，表明系统存在亚稳态行为。
• 理论建模证实，所观测到的亚稳态源于短程位点排斥与长程腔介导相互作用之间的竞争。

实验结果

研究问题

• RQ1 长程相互作用在 destabilize Mott 绝缘体并驱动其向密度波转变中起什么作用？
• RQ2 系统动力学如何依赖于耦合强度的调节速率，以及亚稳态的哪些特征会显现？
• RQ3 什么触发了相变过程中观测到的雪崩隧穿过程？
• RQ4 表面层中的初始密度调制如何导致 Mott 绝缘体核心区域的集体隧穿？
• RQ5 隧穿过程的定量时间尺度和涉及的原子数量是多少？

主要发现

• 系统表现出两种不同的终态：低光子通量的 Mott 绝缘体和高光子通量的密度波，表明该相变由长程相互作用驱动。
• 当全局耦合强度缓慢调节时，观察到迟滞现象，为系统中存在亚稳态提供了直接证据。
• 在相变过程中，数千个原子在单粒子动力学时间尺度内隧穿至相邻晶格位点。
• 初始密度调制形成于被捕获气体可压缩表面层，触发了 Mott 绝缘区域的雪崩隧穿过程。
• 理论建模证实，所观测到的亚稳态源于短程位点排斥与长程腔介导相互作用之间的竞争。
• 开关过程中光子通量的演化呈现快速上升，标志着集体隧穿动力学的开始。