  Quantum dynamics of coherent states is studied within quantum field theory
using two complementary methods: by organizing the evolution as a Taylor series
in elapsed time and by perturbative expansion in coupling within the
interaction-picture formalism. One of the important aspects of our analysis
consists in utilizing the operators and the vacuum of interacting theory in
constructing the states, without invoking asymptotic particles. Focusing on a
coherent state describing a spatially homogeneous field configuration, it is
demonstrated that both adopted methods successfully account for nonlinear
classical dynamics, giving distinguishable quantum effects. In particular,
according to the time-expansion analysis the initial field-acceleration, with
which the field departs from its initial expectation value, is governed by the
tree-level potential with renormalized mass and bare coupling constant. The
interaction-picture computation, instead, can be manipulated to give the
nonlinear dynamics, determined in terms of renormalized coupling and mass.
However, it results in a logarithmic initial-time singularity in the
field-acceleration, reminiscent of the similar behaviour encountered within
semi-classical formalism, for certain choices of the initial state for
fluctuations. Within our coherent-state analysis, the above mentioned
peculiarities are artefacts of an expansion: in the first case over
infinitesimal time, while in the second case in the coupling constant. Despite
this, we show that the evolution obtained within the interaction-picture
analysis is valid for extended period of time. Moreover, on top of the desired
classical dynamics, it serves us with interesting quantum corrections,
previously proposed by Dvali-Gomez-Zell.
