Quantized vortices in an exciton-polariton condensate

One of the most striking quantum effects in an interacting Bose gas at low temperature is superfluidity. First observed in liquid He-4, this phenomenon has been intensively studied in a variety of systems for its remarkable features such as the persistence of superflows and the proliferation of quantized vortices(1). The achievement of Bose-Einstein condensation in dilute atomic gases(2) provided the opportunity to observe and study superfluidity in an extremely clean and well-controlled environment. In the solid state, Bose-Einstein condensation of exciton polaritons has been reported recently(3-6). Polaritons are strongly interacting light-matter quasiparticles that occur naturally in semiconductor microcavities in the strong-coupling regime and constitute an interesting example of composite bosons. Here, we report the observation of spontaneous formation of pinned quantized vortices in the Bose-condensed phase of a polariton fluid. Theoretical insight into the possible origin of such vortices is presented in terms of a generalized Gross-Pitaevskii equation. Whereas the observation of quantized vortices is, in itself, not sufficient for establishing the superfluid nature of the non-equilibrium polariton condensate, it suggests parallels between our system and conventional superfluids.