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Quantum entanglement can be used in a communication scheme to establish acorrelation between successive channel inputs that is impossible by classicalmeans. It is known that the classical capacity of quantum channels can beenhanced 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 classicalcapacity of an entanglement-breaking channel even when it is assisted by aclassical feedback link from the receiver to the transmitter. In doing so, weidentify a bound on the strong converse exponent, which determines theexponentially decaying rate at which the success probability tends to zero, fora sequence of codes with communication rate exceeding capacity. Proving astrong converse, along with an achievability theorem, shows that the classicalcapacity is a sharp boundary between reliable and unreliable communicationregimes. One of the main tools in our proof is the sandwiched Renyi relativeentropy. The same method of proof is used to derive an exponential bound on thesuccess probability when communicating over an arbitrary quantum channelassisted by classical feedback, provided that the transmitter does not useentangled encoding schemes.

Strong converse exponents for the feedback-assisted classical capacity of entanglement-breaking channels