Novel superfluid phenomena in semiconductor microcavities

超流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，并表现为许多迷人的效应，例如超流体通过障碍物的无耗散流动，量子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，后者是某些物质的非线性行为的原因。宏观相干和超流现象不仅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。最近在远离平衡的系统中也观察到了这种现象，在这种系统中，通过驱动和损耗的动态平衡来获得稳态。半导体微腔是目前研究非平衡超流性的重要手段。半导体中玻色子激发与受限光的强相互作用（称为激子）导致新的准粒子-微腔极化激元，其中可以通过改变驱动功率和光子与激子之间的能量失谐来外部操纵相互作用，并且可以通过测量发射光的性质来精确地探测物质成分。半导体微腔的另一个优点是，在目前的实验中，BEC和超流性的温度是10 K的数量级，并且仅受到GaAs中激子和光子之间相对较小的偶极相互作用的限制。其他材料，如GaN，已经在室温下产生极化激元激光，现在只需要在制造更好质量的样品（不均匀无序较少）方面取得技术进步，以便在室温下实现极化激元BEC和超流性。在如此高的温度下的量子集体效应可能会导致器件应用，例如在量子存储和计算中，以及在宏观距离上无损耗地传输光物质脉冲。然而，与热平衡超流体相比，耗散和驱动量子系统中的非平衡超流体的性质是根本不同的，因此，我们面临着一个令人兴奋的机会，去发现和探索全新的物理现象。本计画旨在利用半导体微腔中的极化激元，探索非平衡凝聚体的新超流性质。这是一个非常广泛的主题，因为基本上所有在平衡超流性背景下发现和讨论的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.