  We present the first study of parametric resonance in quantum field theory
from a complete next-to-leading order calculation in a 1/N-expansion of the 2PI
effective action, which includes scattering and memory effects. We present a
complete numerical solution for an O(N)-symmetric scalar theory and provide an
approximate analytic description of the nonlinear dynamics in the entire
amplification range. We find that the classical resonant amplification at early
times is followed by a collective amplification regime with explosive particle
production in a broad momentum range, which is not accessible in a
leading-order calculation.
