Kaiyuan Ji - Quantum Science and Engineering Seminar

Retrocausal Capacity Of A Quantum Channel: Communicating Through Noisy Closed Timelike Curves

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.

Bio: Kaiyuan Ji is a fifth-year PhD student at Cornell University, advised by Mark M. Wilde.  His research concerns characterizing fundamental limits of quantum information-processing tasks from an information-theoretic perspective.  Recent work includes quantum communication through noisy closed timelike curves, quantum thermodynamics with feedback control, and quantum hypothesis testing with inconclusiveness.