  We present a theoretical study of the quantum depletion of microcavity
polaritons that are excited with a resonant laser pulse. The dynamics of the
quantum fluctuations are interpreted in the context of quantum quenches in
general and in terms of the dynamical Casimir effect in particular. We compute
the time evolution of the first and second order correlation functions of the
polariton condensate. Our theoretical modelling is based on the truncated
Wigner approximation for interacting Bose gases. For homogeneous systems,
analytical results are obtained in the linearised Bogoliubov approximation.
Inhomogeneous systems are studied numerically by Monte Carlo simulations.
