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Negative drag in nonequilibrium polariton quantum fluids

The possibility of a negative drag force on a defect in nonequilibrium polariton quantum fluids is presented. We relate this phenomenon to the selective parametric amplification of the waves scattered by the defect. This leads to the prediction that mobile defects acquire a non-zero velocity with respect to the polariton nonequilibrium fluid. We derive a direct relation between the drag force and the momentum distribution of the fluid, that allows for the experimental verification of our predictions.