We reformulate quantum particle creation in time dependent classical
backgrounds using a complete orthonormal family of localized wave packets. The
formalism is then applied to analyze the reheating process after inflation
during which almost all matter populating the universe in the subsequent
radiation era is produced by the decay of the inflaton field. We calculate
the deviations in the number and the energy densities of particles produced
in a volume of a given size and find that there exists (in general tiny)
spatial variations on Hubble length scales over which local interactions are
incapable of restoring homogeneity. This signals the destruction of perfect
homogeneity attained after inflation due to the quantum nature of particle
production process in reheating.