量子气体中湍流级联的出现
Emergence of a Turbulent Cascade in a Quantum Gas
摘要：
在湍流的现代理解中，一个中心概念是存在从大到小的长度尺度的激发级联，反之亦然。这个概念是由Kolmogorov和Obukhov在1941年提出的，并且这个现象已经在各种系统中被观察到，包括行星际等离子体，超新星，海浪和金融市场。尽管取得了很大进展，但由于许多长度尺度的相互作用，对湍流的定量理解仍然是一个挑战，这通常会阻碍对现实实验条件的理论模拟。在这里，我们观察到的湍流级联的出现在一个弱相互作用的均匀玻色气体，量子流体，是服从于所有相关的长度尺度上的理论描述。我们准备了一个玻色-爱因斯坦凝聚体（BEC）在一个光学盒，驱动它的平衡与振荡力，泵入系统的能量在最大的长度尺度，研究BEC的非线性响应的周期性驱动，并观察到一个级联的逐步发展的特点是各向同性幂律分布的动量空间。我们使用Gross-Pitaevskii方程（GPE）对我们的实验进行了数值模拟，并发现与测量结果非常吻合。我们的实验确立了均匀玻色气体作为研究湍流许多方面的一个有前途的新平台，包括涡旋和波湍流的相互作用以及量子和经典效应的相对重要性。
英文摘要：
In the modern understanding of turbulence, a central concept is the existence of cascades of excitations from large to small lengthscales, or vice-versa. This concept was introduced in 1941 by Kolmogorov and Obukhov, and the phenomenon has since been observed in a variety of systems, including interplanetary plasmas, supernovae, ocean waves, and financial markets. Despite a lot of progress, quantitative understanding of turbulence remains a challenge due to the interplay of many lengthscales that usually thwarts theoretical simulations of realistic experimental conditions. Here we observe the emergence of a turbulent cascade in a weakly interacting homogeneous Bose gas, a quantum fluid that is amenable to a theoretical description on all relevant lengthscales. We prepare a Bose-Einstein condensate (BEC) in an optical box, drive it out of equilibrium with an oscillating force that pumps energy into the system at the largest lengthscale, study the BEC's nonlinear response to the periodic drive, and observe a gradual development of a cascade characterised by an isotropic power-law distribution in momentum space. We numerically model our experiments using the Gross-Pitaevskii equation (GPE) and find excellent agreement with the measurements. Our experiments establish the uniform Bose gas as a promising new platform for investigating many aspects of turbulence, including the interplay of vortex and wave turbulence and the relative importance of quantum and classical effects.
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