  We study a quantum model of dynamical Casimir effect in an optical cavity
enclosed by a freely moving mirror attached to a harmonic spring. The quantum
fluctuations of the friction force exerted by the dynamical Casimir emission
onto the moving mirror are investigated, as well as their consequences on the
quantum state of the mirror oscillation. Observable signatures of the interplay
of the nonlinear nature of the effective mirror-cavity coupling and of the
discreteness of the emitted photons are pointed out, in particular as a fast
diffusion of the mirror oscillation phase. These results are interpreted in the
language of quantum field theories on curved space-times as a breakdown of the
standard semiclassical theory of the backreaction.
