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 Rényi 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.

中文翻译：

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纠缠通道的反馈辅助经典能力的强逆

量子纠缠可用于通信方案中，以建立连续通道输入之间的相关性，而这是传统方法无法实现的。已知可以通过这种纠缠的编码方案来增强量子信道的经典容量，但是并非总是如此。在本文中，我们证明了即使在从接收器到发射器的经典反馈链接的帮助下，强逆定理也适用于纠缠信道的经典容量。这样做，我们确定了强逆指数的界限，对于通信速率超过容量的代码序列，该界限确定了成功概率趋于零的指数衰减率。证明一个强有力的反面以及可实现性定理，表明经典能力是可靠和不可靠的交流制度之间的鲜明界限。我们证明的主要工具之一是夹心的Rényi相对熵。如果发射机不使用纠缠编码方案，则在通过经典反馈辅助的任意量子信道上进行通信时，可以使用相同的证明方法来得出成功概率的指数范围。