  Using the Schwinger-Keldysh (closed time path or CTP) and Feynman-Vernon
influence functional formalisms we obtain a Langevin equation for the
description of the charged particle creation in electric field and of
backreaction of charged quantum fields and their fluctuations on time evolution
of this electric field. We obtain an expression for the influence functional in
terms of Bogoliubov coefficients for the case of quantum electrodynamics with
spin 0 charged particles. Then we derive a CTP effective action in
semiclassical approximation and its cumulant expansion. An intimate connection
between CTP effective action and decoherence functional will allow us to
analyze how macroscopic electromagnetic fields are ``measured'' through
interaction with charges and thereby rendered classical.
