Coherently driven microcavity-polaritons and the question of superfluidity

Due to their driven-dissipative nature, photonic quantum fluids present new challenges in understanding superfluidity. Some associated effects have been observed, and notably the report of nearly dissipationless flow for coherently driven microcavity-polaritons was taken as a 'smoking gun' for superflow. Here we show that the superfluid response - the difference between responses to longitudinal and transverse forces - is zero for coherently driven polaritons. This is a direct consequence of the gapped excitation spectrum caused by external phase locking. Furthermore, while a normal component exists at finite pump momentum, the remainder forms a rigid state that does not respond to either longitudinal or transverse perturbations. Interestingly, the total response almost vanishes when the real part of the excitation spectrum has a linear dispersion at low frequency, characteristic of equilibrium bosonic superfluids, which was the regime investigated experimentally. These results suggest that the observed suppression of scattering should be interpreted as a sign of this new rigid state and not of a superfluid.