Entanglement-assisted classical capacity of noisy quantum channels

Prior entanglement between sender and receiver, which exactly doubles the c lassical capacity of a noiseless quantum channel, can increase the classica l capacity of some noisy quantum channels by an arbitrarily large constant factor depending on the channel, relative to the best known classical capac ity achievable without entanglement. The enhancement factor is greatest for very noisy channels, with positive classical capacity but zero quantum cap acity. We obtain exact expressions for the entanglement-assisted capacity o f depolarizing and erasure channels in d dimensions.