Understanding Bose-Einstein Condensation of Light

在过去的一个世纪里，对宏观尺度上的量子效应的探索吸引了许多物理学家。玻色-爱因斯坦凝聚(BEC)是产生这种非凡行为的途径之一。BEC发生在由大量玻色子(遵循玻色-爱因斯坦统计的粒子)组成的系统中，例如光子(光的量子化粒子)和具有偶数个组成部分的原子。当气体的温度变得如此之低，以至于能量分布遵循量子力学规则的唯一方式是通过转变到许多粒子处于单一最低能量量子态的阶段时，就形成了BEC。这导致气体的性质发生了戏剧性的变化，因为整个系综表现为单个量子粒子。最初人们认为，由于粒子的数量不守恒，所以永远不会形成光子的BEC，因此当光子气体冷却时，粒子被容器吸收，气体不会凝结。值得注意的是，在2010年，也就是相关物理学首次被讨论的大约100年后，这个问题被克服了，并通过实验观察到了室温下光子的BEC。这一成功打开了一系列全新的问题，必须加以解决，以最大限度地发挥未来实验的潜力。虽然这些系统与在超冷原子中观察到的传统平衡BEC有许多相似之处，但由于粒子的有限寿命，它们有很大的不同。这导致了丰富多样的物理行为，包括非平衡超流、自发的量化涡旋和其他我打算探索的奇异现象。我之前的工作已经解决了最新的实验结果。然而，人们对将这些系统用作理解多体量子现象(需要许多相互作用的粒子的复杂行为)的工具箱非常感兴趣。对于这些应用程序，到目前为止开发的工具是不够的。像这样的系统，在相干量子效应和损耗之间具有强烈的耦合和竞争，无论是在分析上还是在数值上，都是非常具有挑战性的。我打算在这里开发的工具将为理解这些室温量子相干系统的行为提供一条途径。

The search for quantum effects on macroscopic scales has fascinated many physicists over the past century. Bose-Einstein condensation (BEC) is one route to such remarkable behaviour. BEC occurs in systems formed from large collections of bosons (particles which follow Bose-Einstein statistics), such as photons (the quantised particle of light), and atoms with an even number of constituent parts. A BEC forms when the temperature of a gas becomes so low that the only way the energy distribution can follow the rules of quantum mechanics is by transitioning to a phase in which many of the particles are in the single lowest energy quantum state. This causes a dramatic change in the properties of the gas as the whole ensemble behaves as a single quantum particle. Initially it was thought that a BEC of photons could never form since the number of particles is not conserved, and so as a gas of photons is cooled the particles are absorbed by the container, and the gas does not condense. Remarkably, in 2010, around 100 years since the relevant physics was first discussed, this problem was overcome and the room temperature BEC of photons was experimentally observed. This success has opened up a whole new set of questions which must be addressed in order to maximise the potential for future experiments. These systems, while sharing many similarities with the conventional equilibrium BECs observed in ultra-cold atoms, are very different due to the finite lifetime of the particles. This gives rise to a rich variety of physical behaviour including non-equilibrium superfluidity, spontaneous quantised vortices and other exotic phenomena which I intend to explore. My previous work has addressed the latest experimental results. However, there is significant interest in using these systems as a toolbox for understanding many-body quantum phenomena (complex behaviour requiring many interacting particles). For these applications the tools developed so far are inadequate. Systems such as these, with strong coupling and competition between coherent quantum effects and losses, are very challenging to treat, analytically or numerically. The tools I propose to develop here will provide a route to understanding the behaviour of these room temperature quantum coherent systems.