Enhanced communication with the assistance of indefinite causal order

TL;DR

This paper demonstrates that quantum channels combined in a superposition of orders can enable communication tasks impossible in classical arrangements, challenging traditional assumptions in quantum Shannon theory.

Contribution

It introduces the concept of quantum superpositions of channel orders, showing they can unlock communication capabilities in noisy quantum networks.

Findings

Two depolarizing channels can transmit information when combined in superposition.

Superposition of channel orders can outperform classical arrangements in quantum communication.

Identical channels can have different capacities depending on their quantum order.

Abstract

In quantum Shannon theory, the way information is encoded and decoded takes advantage of the laws of quantum mechanics, while the way communication channels are interlinked is assumed to be classical. In this Letter we relax the assumption that quantum channels are combined classically, showing that a quantum communication network where quantum channels are combined in a superposition of different orders can achieve tasks that are impossible in conventional quantum Shannon theory. In particular, we show that two identical copies of a completely depolarizing channel become able to transmit information when they are combined in a quantum superposition of two alternative orders. This finding runs counter to the intuition that if two communication channels are identical, using them in different orders should not make any difference. The failure of such intuition stems from the fact that a single noisy channel can be a random mixture of elementary, non-commuting processes, whose order (or lack thereof) can affect the ability to transmit information.