### Abstract

Classical back communication does not increase a classical channel's forward capacity, but for quantum channels the situation is more complicated. It is well known that classical back communication can increase quantum capacity, but no example was known where it increased a quantum channel's classical capacity. We have found channels whose Holevo capacity is almost zero, but whose quantum and classical capacities in the presence of classical back communication are quite large. The greater complexity of the situation with quantum channels stems from the fundamental difference between entanglement and its closest classical analog, shared secret randomness. Joint work with Igor Devetak, Peter Shor, and John Smolin. quant-ph/0406086.