Bogoliubov dispersion relation for a 'photon fluid': Is this a superfluid?

We discuss the possibility that photons. which are bosons, can form a 2D superfluid due to Bose-Einstein condensation inside a nonlinear Fabry-Perot cavity filled with atoms in their ground states. A 'photon fluid' forms inside the cavity as a result of multiple photon-photon collisions mediated by the atoms during a cavity ring-down time. The effective mass and chemical potential for a photon inside this fluid are nonvanishing. This implies the existence of a Bogoliubov dispersion relation for the low-lying elementary excitations of the photon fluid, and in particular, that sound waves exist for long-wavelength, low-frequency disturbances of this fluid. Possible experiments to test for the superfluidity of the photon fluid based on the Landau critical-velocity criterion will be discussed. (C) 2000 Elsevier Science B.V. All rights reserved.