  Quantum entanglement can be used in a communication scheme to establish a
correlation between successive channel inputs that is impossible by classical
means. It is known that the classical capacity of quantum channels can be
enhanced by such entangled encoding schemes, but this is not always the case.
In this paper, we prove that a strong converse theorem holds for the classical
capacity of an entanglement-breaking channel even when it is assisted by a
classical feedback link from the receiver to the transmitter. In doing so, we
identify a bound on the strong converse exponent, which determines the
exponentially decaying rate at which the success probability tends to zero, for
a sequence of codes with communication rate exceeding capacity. Proving a
strong converse, along with an achievability theorem, shows that the classical
capacity is a sharp boundary between reliable and unreliable communication
regimes. One of the main tools in our proof is the sandwiched Renyi relative
entropy. The same method of proof is used to derive an exponential bound on the
success probability when communicating over an arbitrary quantum channel
assisted by classical feedback, provided that the transmitter does not use
entangled encoding schemes.
