We study the capacity of a quantum channel for retrocausal communication, where messages are transmitted backward in time, from a sender in the future to a receiver in the past, through a noisy postselected closed timelike curve mathematically represented by the channel. We completely characterize the one-shot retrocausal quantum and classical capacities, and we show that the corresponding asymptotic capacities are equal to the average and sum, respectively, of the channel's max-information and its regularized Doeblin information. This endows these information measures with a novel operational interpretation. Furthermore, our characterization can be generalized beyond quantum channels to all completely positive maps. This imposes information-theoretic limits on transmitting messages via postselected-teleportation-like mechanisms with arbitrary initial- and final-state boundary conditions, including those considered in various black-hole final-state models.

翻译：我们研究了量子信道用于回溯因果通信的容量，其中信息通过数学上由该信道表示的含噪后选择闭合类时曲线，从未来发送者反向传输至过去接收者。我们完整刻画了单次回溯因果量子容量和经典容量，并证明相应的渐近容量分别等于信道的最大信息量及其正则化Doeblin信息的平均值与总和。这为这些信息度量赋予了全新的操作解释。此外，我们的刻画可超越量子信道推广至所有完全正映射。这为通过后选择隐形传态类机制（具有任意初末态边界条件，包括各类黑洞末态模型中考虑的边界条件）传输消息施加了信息论极限。