  Quantum communication schemes widely use dielectric four-port devices as
basic elements for constructing optical quantum channels. Since for causality
reasons the permittivity is necessarily a complex function of frequency,
dielectrics are typical examples of noisy quantum channels in which quantum
coherence will not be preserved. Basing on quantization of the phenomenological
electrodynamics, we construct the transformation relating the output quantum
state to the input quantum state without placing frequency restrictions.
Knowledge of the full transformed quantum state enables us to compute the
entanglement contained in the output quantum state. We apply the formalism to
some typical examples in quantum communication.
