  The processes of radiation damping and decoherence in Quantum Electrodynamics
are studied from an open system's point of view. Employing functional
techniques of field theory, the degrees of freedom of the radiation field are
eliminated to obtain the influence phase functional which describes the reduced
dynamics of the matter variables. The general theory is applied to the dynamics
of a single electron in the radiation field. From a study of the wave packet
dynamics a quantitative measure for the degree of decoherence, the decoherence
function, is deduced. The latter is shown to describe the emergence of
decoherence through the emission of bremsstrahlung caused by the relative
motion of interfering wave packets. It is argued that this mechanism is the
most fundamental process in Quantum Electrodynamics leading to the destruction
of coherence, since it dominates for short times and because it is at work even
in the electromagnetic field vacuum at zero temperature. It turns out that
decoherence trough bremsstrahlung is very small for single electrons but
extremely large for superpositions of many-particle states.
