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Investigating non-equilibrium physics and universality using two-dimensional quantum gases

使用二维量子气体研究非平衡物理和普遍性

基本信息

批准号：
EP/S013105/1
负责人：
Christopher Foot
金额：
$ 56.41万
依托单位：
University of Oxford
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2018
资助国家：
英国
起止时间：
2018 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=EP%2FS013105%2F1
关键词：
Investigating non equilibrium physics universality

项目摘要

Systems that are not in equilibrium are ubiquitous but can be complex to describe. Although systems at equilibrium are described with great success by quantum mechanics there is, as yet, no general theoretical framework for how a closed many-body quantum system evolves to such thermalised states. This project investigates the process by which non-equilibrium (NEQ) systems relax towards thermal equilibrium, which we call thermalisation. Macroscopic examples range from the cooling of a cup of coffee to the emergence of structures in the early universe. NEQ processes are also important for quantum systems including quantum computers and quantum heat engines. Our experimental techniques allow many-body quantum systems to be prepared in precisely defined NEQ situations and then track their evolution towards equilibrium in unprecedented level of detail.The system that we will use to gain a better understanding of NEQ physics is a two-dimensional (2D) gas of atoms at temperatures of tens of nanokelvin. The properties of 2D systems are of central importance in physics and part of the Nobel prize for Physics (2016) was awarded to Kosterlitz and Thouless for their work on a phase transition in 2D quantum systems that is named after them. This transition occurs as the quantum gas is cooled and at a certain temperature changes into a superfluid, which flows without friction amongst other fascinating properties. The ultracold atoms are trapped in extremely well-controlled conditions thus enabling us to make definitive quantitative comparisons with theoretical expectations. Quantum systems confined to 2D are especially interesting for studying NEQ processes because the fluctuations, that are an inherent part of quantum mechanics, play a large role in preventing true long-range order. This new method will provide insights into similar phase transitions in other 2D systems such as thin-film superconductors and liquid crystals, and the quantum gas acts as a quantum simulator for 2D quantum physics in general.A cornerstone of this proposal is the double-well potential for ultracold rubidium atoms that we have created recently by an innovative use of combined radio-frequency (RF) and static magnetic fields. This technique is ideally suited for coherent splitting of a 2D quantum gas because the shape and height of the potential are controlled directly by the applied RF fields, thus exploiting the extremely high precision of RF electronics. The rate of splitting determines the energy deposited into the system to produce a chosen initial state. At a predefined time after the splitting, the two clouds are released from the double-well potential so that they expand and overlap. This permits interferometric measurements of the relative phase of the matter waves. From repeated measurements, each with the initial state prepared in the same way, we will be able to determine the probability distribution function (PDF) corresponding to the relative phase of the quantum gas for all positions in the 2D plane. PDFs represent the essence of quantum mechanics and allow a more comprehensive comparison with theoretical models than monitoring the time evolution of the expectation values of certain observables as is commonly done. This cold-atom apparatus acts as a 'quantum simulator' of many-body phases in 2D systems thus providing fresh insights relevant to long-standing research questions.

不平衡的系统是普遍存在的，但描述起来可能很复杂。虽然量子力学成功地描述了处于平衡状态的系统，但对于封闭的多体量子系统如何演化到这种热态，还没有通用的理论框架。本项目研究非平衡（NEQ）系统向热平衡放松的过程，我们称之为热化。宏观的例子从一杯咖啡的冷却到早期宇宙结构的出现。NEQ过程对于包括量子计算机和量子热机在内的量子系统也很重要。我们的实验技术允许在精确定义的NEQ情况下制备多体量子系统，然后以前所未有的细节水平跟踪它们向平衡的演化。我们将用来更好地理解NEQ物理的系统是一个温度为数十纳凯的二维（2D）原子气体。2D系统的性质在物理学中具有核心重要性，2016年诺贝尔物理学奖的一部分授予了Kosterlitz和Elderless，以表彰他们在以他们命名的2D量子系统中的相变方面的工作。这种转变发生在量子气体冷却并在一定温度下变成超流体时，这种超流体在其他迷人的特性中没有摩擦。超冷原子被困在非常良好的控制条件下，从而使我们能够与理论预期进行明确的定量比较。局限于2D的量子系统对于研究NEQ过程特别有趣，因为作为量子力学固有部分的涨落在阻止真正的长程有序方面发挥了很大作用。这种新方法将为其他二维系统（如薄膜超导体和液晶）中类似的相变提供见解，而量子气体通常充当二维量子物理的量子模拟器。该提议的基石是我们最近通过组合射频（RF）和静态磁场的创新使用创建的超冷铷原子的双阱势。这种技术非常适合2D量子气体的相干分裂，因为电势的形状和高度直接由施加的RF场控制，从而利用了RF电子器件的极高精度。分裂的速率决定了储存到系统中的能量，以产生选定的初始状态。在分裂之后的预定义时间，两个云从双阱势释放，使得它们扩展并重叠。这使得干涉测量的相对相位的物质波。通过重复的测量，每次测量都以相同的方式准备初始状态，我们将能够确定对应于2D平面中所有位置的量子气体相对相位的概率分布函数（PDF）。PDF代表了量子力学的本质，并且允许与理论模型进行更全面的比较，而不是像通常那样监视某些可观测值的期望值的时间演化。这种冷原子装置作为2D系统中多体相的“量子模拟器”，从而为长期存在的研究问题提供了新的见解。