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arXiv · 2509.08965

Retrocausal capacity of a quantum channel: Communicating through noisy closed timelike curves

Kaiyuan Ji·Seth Lloyd·Mark M. Wilde
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Abstract

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.

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Kaiyuan Ji, Seth Lloyd, Mark M. Wilde. 2025-09-10. Retrocausal capacity of a quantum channel: Communicating through noisy closed timelike curves. https://doi.org/10.1103/znyd-npk5

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