  We show that for the tensor product of an entanglement-breaking quantum
channel with an arbitrary quantum channel, both the minimum entropy of an
output of the channel and the Holevo-Schumacher-Westmoreland capacity are
additive. In addition, for the tensor product of two arbitrary quantum
channels, we give a bound involving entanglement of formation for the amount of
subadditivity (for minimum entropy output) or superadditivity (for channel
capacity) that can occur.

Quantum Physics