Superfluidity, the ability of a quantum fluid to flow without friction, is one of the most spectacular phenomena occurring in degenerate gases of interacting bosons. Since its first discovery in liquid helium-4 (refs 1, 2), superfluidity has been observed in quite different systems, and recent experiments with ultracold trapped atoms have explored the subtle links between superfluidity and Bose-Einstein condensation 3-5 . In solid-state systems, it has been anticipated that excitonpolaritons in semiconductor microcavities should behave as an unusual quantum fluid 6-8 , with unique properties stemming from its intrinsically non-equilibrium nature. This has stimulated the quest for an experimental demonstration of superfluidity effects in polariton systems 9-13 . Here, we report clear evidence for superfluid motion of polaritons. Superfluidity is investigated in terms of the Landau criterion and manifests itself as the suppression of scattering from defects when the flow velocity is slower than the speed of sound in the fluid. Moreover, aČerenkov-like wake pattern is observed when the flow velocity exceeds the speed of sound. The experimental findings are in quantitative agreement with predictions based on a generalized Gross-Pitaevskii theory 12,13 , and establish microcavity polaritons as a system for exploring the rich physics of non-equilibrium quantum fluids.Bound electron-hole particles, known as excitons, are fascinating objects in semiconductor nanostructures. In a quantum well with a thickness of the order of a few nanometres, the external motion of the exciton is quantized in the direction perpendicular to the well, whereas it is free within the plane of the well. When the quantum well is placed in a high-finesse microcavity, the strong-coupling regime between excitons and light is easily reached 14 , giving rise to exciton-photon mixed quasiparticles called polaritons, which are an interesting kind of two-dimensional composite boson. Thanks to their sharp dispersion, polaritons have a small effective mass (of the order of 10 −5 times the free-electron mass) that allows the building of many-body coherent effects, such as Bose-Einstein condensation 15,16 , at a lattice temperature of a few kelvins. Furthermore, their partially excitonic character results in strong interactions between polaritons, which are expected to lead to the appearance of superfluid phenomena. Indirect evidence of superfluid motion in polariton systems has recently been reported through the observation of pinned quantized vortices 9 , Bogoliubov-like dispersions 10 and pioneering experiments on polariton parametric oscillators 11 . Despite these remarkable works, a direct demonstration of exciton-polariton superfluidity is however still missing. In this Letter, we report the observation of superfluid motion of a quantum fluid of polaritons created by a laser in a semiconductor microcavity.In our experiments, to probe superfluidity we study the perturbation that is produced in an optically created moving polariton fluid when a static ...

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