Entanglement transformation at dielectric four-port devices
  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. 
S.Scheel, L.Knoll, T.Opatrny, and D.-G.Welsch