  Classical feedback is defined here as the knowledge by the transmitter of the
quantum state of the qubit received by the receiver. Such classical feedback
doubles capacities of certain memoryless quantum channels without preexisting
entanglement between transmitter and receiver. The increase in capacity, which
is absent on classical memoryless channels, occurs because we can transform an
entangled qubit pair into any other entangled state by applying a unitary
operator to only one of the qubits.
