Understanding Bose-Einstein Condensation of Light

在过去的世纪里，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）是这种非凡行为的一种途径。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法是过渡到一个阶段，其中许多粒子处于单一的最低能量量子态。这导致了气体性质的巨大变化，因为整个系综表现为单个量子粒子。最初，人们认为光子的BEC永远不会形成，因为粒子的数量不守恒，所以当光子气体冷却时，粒子被容器吸收，气体不会冷凝。值得注意的是，在2010年，自相关物理学首次讨论以来约100年，这个问题被克服，并在实验中观察到光子的室温BEC。这一成功开启了一系列全新的问题，为了最大限度地发挥未来实验的潜力，必须解决这些问题。这些系统虽然与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而非常不同。这引起了丰富多样的物理行为，包括非平衡超流，自发量子涡旋和其他奇异现象，我打算探讨。我以前的工作已经解决了最新的实验结果。然而，人们对使用这些系统作为理解多体量子现象（需要许多相互作用粒子的复杂行为）的工具箱非常感兴趣。对于这些应用，迄今为止开发的工具是不够的。像这样的系统，具有强耦合和竞争之间的相干量子效应和损失，是非常具有挑战性的处理，分析或数值。我建议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.