Dilute Quantum Fluids Beyond the Mean-Field

如果我们深入到自然界的微观层面，我们会发现一个由量子力学支配的奇怪世界，我们的直觉被打破了。在这种令人着迷的状态下，粒子的位置具有内在的不确定性，并且一直在波动。这种量子涨落是许多物理现象的核心，从范德华力到黑洞中的霍金辐射，可能会为基于量子效应的技术提供终极限制。然而，量子涨落很难从实验上观察到，也很难在理论上描述。自1995年实现量子涨落以来，玻色-爱因斯坦凝聚体(BEC)提供了一个独特的窗口来观察量子世界。BEC是一种相同原子的气体，冷却到绝对零度之上不到百万分之一度。在这一点上，单个原子位置的不确定性变得大于原子之间的分离，因此不可能识别单个原子。相反，这种气体的行为就像受量子力学支配的巨大物质波，并显示出一系列引人注目的量子性质，如干扰另一种BEC的能力和无粘性(超流)流动的能力。此外，BEC可以接受高度的实验控制(例如，在时间和空间上操纵和询问系统)，并且它们可以以高分辨率成像。BEC的行为，包括上面的属性，通过只考虑所有原子的平均行为--所谓的“平均场”--来高度准确地捕捉到。自1995年以来的几年里，实验和理论工作的协同作用已经建立了对量子平均场及其如何影响系统行为的深刻理解。然而，在BEC中，与平均场相比，量子涨落很小，因此，BEC的优点并没有扩展到量子涨落的领域。当两个BEC共存时，可以使每个BEC的平均场量子效应相互抵消，留下量子涨落作为系统内的主导效应。这导致系统从BEC气体变为液滴。但这远远不同于传统的液滴：比方说，水很难压缩，因为邻近原子的电子壳层拒绝重叠，而在量子液体中，这是因为量子涨落。因此，量子水滴的存在归功于本质上的量子效应；这使得它成为一个迷人的研究对象。此外，它还提供了一个研究量子涨落的平台，从微观起源到宏观表现。我们将在英国首次使用铯和YbBEC的混合物来设计量子液滴；这种原子组合将使我们能够对液体施加高水平的控制。鉴于这种状态是最近才被发现的，有很多东西需要研究和学习。我们将利用我们的实验能力将水滴推向极限。我们将制定出它们得到支持的政权，以及它们如何形成的细节。我们将在一系列场景中实验性地询问他们，有效地刺激和推动他们，以了解他们是如何反应的。我们将特别关注有效的2D和1D几何结构，在这些几何结构中，量子涨落预计将大大增强。除了我们的实验，我们还将开发和测试理论模型来描述我们的观察结果；这将使我们能够解决有关潜在物理的公开问题，并量化量子涨落的准确作用。我们的工作结果将对加深我们对量子涨落的理解具有基本的重要性，并可能促进量子液滴的应用，如在精密光谱和沉积方面。

If we peer deep inside nature to a microscopic level, we find a strange world governed by quantum mechanics where our intuition breaks down. In this fascinating regime, the position of a particle has inherent uncertainty and is perpetually fluctuating. Such quantum fluctuations lie at the heart of a number of physical phenomena, ranging from the van der Waals force to Hawking radiation in black holes, and may provide the ultimate limit to technologies based on quantum effects. However, quantum fluctuations are difficult to observe experimentally and to describe theoretically.Since their realization in 1995, Bose-Einstein condensates (BECs) have provided a unique window through which to view the quantum world. A BEC is a gas of identical atoms cooled down to less than a millionth of a degree above absolute zero. At this point the uncertainty in an individual atom's position becomes greater than the separation between atoms and it is impossible to identify individual atoms. Instead, the gas behaves like a giant wave of matter dominated by quantum mechanics, and displays a range of striking quantum properties such as the ability to interfere with another BEC and the ability to flow without viscosity (superfluidity). In addition, BECs are amenable to a high degree of experimental control (for example, to manipulate and interrogate the system in time and space) and they can be imaged to high resolution. The behaviour of BECs, including the properties above, are captured to a high degree of accuracy by considering just the average behaviour of all the atoms: the so-called "mean-field". Over the years since 1995, a synergy of experiments and theoretical works have established a deep understanding of the quantum mean-field and how it influences the system behaviour. However, in a BEC, quantum fluctuations are small compared to the mean-field, and as such, the merits offered by BECs have not extended to the realm of quantum fluctuations.Enter the quantum liquid droplet. When two BECs co-exist, the mean-field quantum effects from each BEC can be made to cancel each other out, leaving behind the quantum fluctuations as the dominant effect within the system. This causes the system to change from a BEC gas to a liquid-like droplet. But this is far from your conventional liquid droplet: whereas, say, water is hard to compress because the electronic shells of neighbouring atoms refuse to overlap, in the quantum liquid it is because of quantum fluctuations. As such, the quantum droplet owes its existence to intrinsically quantum effects; this makes it a fascinating object to study. Moreover, it provides a platform to study quantum fluctuations, from their microscopic origins to their macroscopic manifestations.We will engineer quantum droplets, for the first time in the UK, using a mixture of caesium and ytterbium BECs; this atomic combination will enable us to exert high levels of control over the liquid. Given that this state has only recently been discovered, there is much to study and learn. We will use our experimental capabilities to push the droplets to their limits. We will map out the regimes for which they are supported, as well as the details of how they form. We will experimentally interrogate them in a range of scenarios, effectively prodding and pushing them, to understand how they respond. We will pay particular attention to effectively 2D and 1D geometries where quantum fluctuations are predicted to be greatly enhanced. Alongside our experiments, we will develop and test theoretical models to describe our observations; this will allow us to address open questions regarding the underlying physics and quantify the precise role of the quantum fluctuations. The findings of our work will be of fundamental importance in deepening our understanding of quantum fluctuations and may motivate applications of quantum droplets such as in precision spectroscopy and deposition.