  We define "coherent communication" in terms of a simple primitive, show it is
equivalent to the ability to send a classical message with a unitary or
isometric operation, and use it to relate other resources in quantum
information theory. Using coherent communication, we are able to generalize
super-dense coding to prepare arbitrary quantum states instead of only
classical messages. We also derive single-letter formulae for the classical and
quantum capacities of a bipartite unitary gate assisted by an arbitrary fixed
amount of entanglement per use.
