Quantum and Classical Data Transmission through Completely Depolarising Channels in a Superposition of Cyclic Orders

TL;DR

This paper demonstrates that placing multiple depolarising channels in a superposition of causal orders enables high-fidelity quantum communication and increases classical capacity, revealing complex correlation patterns.

Contribution

It introduces a novel method of using superpositions of N causal orders to enable quantum information transmission through depolarising channels, surpassing previous limitations.

Findings

High-fidelity quantum transmission with error decreasing as 1/N

Classical capacity increases with the number of channels N

Superpositions of causal orders create complex correlation patterns

Abstract

Completely depolarising channels are often regarded as the prototype of physical processes that are useless for communication: any message that passes through them along a well-defined trajectory is completely erased. When two such channels are used in a quantum superposition of two alternative orders, they become able to transmit some amount of classical information, but still no quantum information can pass through them. Here we show that the ability to place N completely depolarising channels in a superposition of N alternative causal orders enables a high-fidelity, heralded transmission of quantum information with error vanishing as 1/N. This phenomenon highlights a fundamental difference with the N = 2 case, where completely depolarising channels are unable to transmit quantum data, even when placed in a superposition of causal orders. The ability to place quantum channels in a superposition of orders also leads to an increase of the classical communication capacity with N, which we rigorously prove by deriving an exact single-letter expression. Our results highlight the more complex patterns of correlations arising from multiple causal orders, which are similar to the more complex patterns of entanglement arising in multipartite quantum systems.