  In this work we study how quantum fluctuations modify the quantum evolution
of an initially classical field theory. We consider a scalar $\phi^4$ theory
coupled to an external source as a toy model for the Color Glass Condensate
description of the early time dynamics of heavy-ion collisions. We demonstrate
that quantum fluctuations considerably modify the time evolution driving the
system to evolve in accordance with ideal hydrodynamics. We attempt to
understand the mechanism behind this relaxation to ideal hydrodynamics by using
modified initial spectra and studying the particle content of the theory.
