Novel superfluid phenomena in semiconductor microcavities

超流动性是相互作用凝聚态系统中宏观量子相干性最显著的结果之一，并表现在许多令人着迷的效应中，例如超流体通过障碍物的无耗散流动、量子化循环和亚稳态持续电流。1937年首次在液氦中观察到，它与玻色-爱因斯坦凝聚（BEC）现象和费米子的Bardeen-Cooper-Schrieffer （BCS）集体态密切相关，后者是导致某些材料超导行为的原因。宏观相干性和超流动性现象并不局限于接近热力学平衡的系统，如液氦、超导体和超冷原子气体。它最近也在远离平衡的系统中被观察到，其中稳态是通过驱动和损失的动态平衡获得的。半导体微腔目前在非平衡超流体的研究中起着主导作用。半导体中受限光和玻色子激发之间的强相互作用（称为激子）导致新的准粒子-微腔极化子，其中的相互作用可以通过改变光子和激子之间的驱动功率和能量失谐来操纵外部，并且可以通过测量发射光的性质来精确探测物质成分。半导体微腔的另一个优点是，在目前的实验中，BEC和超流体的温度约为10K，并且仅受GaAs中激子和光子之间相对较小的偶极子相互作用的限制。其他材料，如氮化镓，已经在室温下进行了极化激子激光，现在的问题只是技术进步，以制造质量更好的样品（更少的非均匀无序），使极化激子BEC和超流体在室温下实现。如此高温下的量子集体效应可能会导致器件应用，例如量子存储和计算，以及在宏观距离上无损耗地传输光物质脉冲。然而，耗散和驱动量子系统中非平衡超流体的特性与热平衡超流体有着根本的不同，因此我们面临着发现和探索全新物理现象的激动人心的机会。本项目旨在利用半导体微腔中的极化子，探索非平衡冷凝物的新型超流体性质。这是一个非常广泛的主题，因为基本上在平衡超流动性的背景下发现和讨论的所有现象都可能受到非平衡的影响。我们的目标是提供一个全面的理论描述，并通过我们的项目合作伙伴的实验实现，与超流体行为有关的广泛现象。

Superfluidity is one of the most remarkable consequences of macroscopic quantum coherence in interacting condensed matter systems, and manifests itself in a number of fascinating effects, such as for example dissipationless flow of a superfluid via obstacles, quantised circulation, and metastable persistent currents. First observed in liquid Helium in 1937, it is closely related to the phenomena of Bose-Einstein condensation (BEC), and to the Bardeen-Cooper-Schrieffer (BCS) collective state of fermions, which is responsible for superconductive behaviour of some materials.The phenomenon of macroscopic coherence and superfluidity is not restricted to systems close to thermodynamic equilibrium, such as liquid Helium, superconductors and ultra-cold atomic gases. It has also been recently observed in systems far from equilibrium, where a steady-state is obtained by a dynamical balance of driving and losses. Semiconductor microcavities currently play a leading role in the study of non-equilibrium superfluidity. Strong interactions between confined light and bosonic excitations in the semiconductor (called excitons) lead to new quasi-particles - microcavity polaritons, in which interactions can be manipulated externally by changing the driving power and the energy detuning between photons and excitons, and where the matter component can be accurately probed by measuring the properties of the emitted light.An additional advantage of semiconductor microcavities is that the temperatures for BEC and superfluidity in current experiments are of the order of 10K, and are only limited by relatively small dipole interactions between excitons and photons in GaAs. Other materials, such as GaN, have already hosted polariton lasing at room temperature, and it is now only a question of technological progress in manufacturing samples of a better quality (less of the inhomogeneous disorder) for polariton BEC and superfluidity to be realised at room temperature. Quantum collective effects at such high temperatures are likely to lead to device applications, for example in quantum storage and computation, and for transporting light-matter pulses without loss over macroscopic distances.However, properties which characterise non-equilibrium superfluids in dissipative and driven quantum systems are fundamentally different compared to superfluids in thermal equilibrium, and thus we are faced with an exciting opportunity to discover and explore brand-new physical phenomena. This project is aimed at exploring novel superfluid properties of non-equilibrium condensates, using polaritons in semiconductor microcavities. It is a very broad subject since essentially all phenomena discovered and discussed in the context of equilibrium superfluidity are likely to be affected by non-equilibrium. Our aim is to provide a comprehensive theoretical description, and experimental realisation by our project partners, of a broad range of phenomena connected with superfluid behaviour.