  This paper addresses some general questions of quantum information theory
arising from the transmission of quantum entanglement through (possibly noisy)
quantum channels. A pure entangled state is prepared of a pair of systems $R$
and $Q$, after which $Q$ is subjected to a dynamical evolution given by the
superoperator $\superop^{Q}$. Two interesting quantities can be defined for
this process: the entanglement fidelity $F_{e}$ and the entropy production
$S_{e}$. It turns out that neither of these quantities depends in any way on
the system $R$, but only on the initial state and dynamical evolution of $Q$.
$F_{e}$ and $S_{e}$ are related to various other fidelities and entropies, and
are connected by an inequality reminiscent of the Fano inequality of classical
information theory. Some insight can be gained from these techniques into the
security of quantum cryptographic protocols and the nature of quantum
error-correcting codes.
