  We report the observation of low-energy, low-momenta collective oscillations
of an excitonpolariton condensate in a round "box" trap. The oscillations are
dominated by the dipole and breathing modes, and the ratio of the frequencies
of the two modes is consistent with that of a weakly interacting
two-dimensional trapped Bose gas. The speed of sound extracted from the dipole
oscillation frequency is smaller than the Bogoliubov sound, which can be partly
explained by the influence of the incoherent reservoir. These results pave the
way for understanding the effects of reservoir, dissipation, energy relaxation,
and finite temperature on the superfluid properties of exciton-polariton
condensates and other two-dimensional open-dissipative quantum fluids.
