Quantum coherence in many-body physics is at the origin of striking effects such as Bose Einstein condensation and superfluidity, observed in liquid Helium and in ultra-cold atomic ensembles. In an optical cavity or in paraxial propagation, propagating photons can behave like matter particles, because they acquire a small effective mass. Moreover they interact with each other if the medium where they propagate contains a nonlinear material. A few years ago, condensation and superfluid behaviour of photons were observed in optical cavities and also in nonlinear media without cavity. This lecture will show how these properties have been studied in a semiconductor microcavity and in a hot rubidium vapour, with the appearance of superfluidity, Cerenkov waves and quantum turbulence with quantized vortices, demonstrating that photons constitute a very rich system for exploring the physics of quantum fluids.
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