  We analyse the quantum mechanical limits to the plasmon-assisted entanglement
transfer observed by E. Altewischer, M.P. van Exter, and J.P. Woerdman [Nature,
418, 304 (2002)]. The maximal violation S of Bell's inequality at the
photodetectors behind two linear media (such as the perforated metal films in
the experiment) can be described by two ratio's tau_1, tau_2 of
polarization-dependent transmission probabilities. A fully entangled incident
state is transferred without degradation for tau_1=tau_2, but a relatively
large mismatch of tau_1 and tau_2 can be tolerated with a small reduction of S.
We predict that fully entangled Bell pairs can be distilled out of partially
entangled radiation if tau_1 and tau_2 satisfy a pair of inequalities.
