  We derive the general formula for the capacity of a noiseless quantum channel
assisted by an arbitrary amount of noisy entanglement. In this capacity
formula, the ratio of the quantum mutual information and the von Neumann
entropy of the sender's share of the noisy entanglement plays the role of
mutual information in the completely classical case. A consequence of our
results is that bound entangled states cannot increase the capacity of a
noiseless quantum channel.
