Polaritons are half-matter half-light quasiparticles formed from the strong coupling between excitons and photons, in systems ranging from semiconductor microcavities to atomic gases. In microcavities, it is established that a gas of polaritons undergoes a phase transition to a Bose-Einstein condensed state with a macroscopic number of particles occupying a single energy level. A key question concerns the extent to which superfluid behavior, including dissipationless transport and non-classical inertia, is present. I will describe our recent theory [Alexander Janot, Timo Hyart, Paul R. Eastham and Bernd Rosenow, arXiv:1307.1407] of the superfluid stiffness -- a quantitative measure of superfluidity accessible in experiments -- for polaritons in a random potential. For an equilibrium condensate superfluidity survives in the presence of a weak random potential, while a strong random potential causes a phase transition to a Bose glass. In contrast, we predict that polaritons never form a superfluid in a random potential, no matter how weak it may be. This reflects the off-equilibrium nature of the condensate. The universal properties of such off-equilibrium Bose-Einstein condensates are therefore fundamentally different from those of equilibrium ones.
Janot et. al. Phys. Rev. Lett. 111, 230403 (2013)