  We show that the amount of coherent quantum information that can be reliably
transmitted down a dephasing channel with memory is maximized by separable
input states. In particular, we model the channel as a Markov chain or a
multimode environment of oscillators. While in the first model the maximization
is achieved for the maximally mixed input state, in the latter it is convenient
to exploit the presence of a decoherence-protected subspace generated by memory
effects. We explicitly compute the quantum channel capacity for the first model
while numerical simulations suggest a lower bound for the latter. In both cases
memory effects enhance the coherent information. We present results valid for
arbitrary size of the input.
