Indefinite causal order enables perfect quantum communication with zero capacity channels

TL;DR

This paper demonstrates that indefinite causal order can enable perfect quantum communication through channels that are individually incapable of transmitting quantum information, revealing a fundamental difference between temporal and spatial superpositions.

Contribution

It shows that superposing the order of independent noisy channels can create a perfect quantum communication channel, a novel phenomenon not achievable with superpositions of paths.

Findings

Indefinite causal order allows perfect quantum communication with zero-capacity channels.

Superposition of alternative orders differs fundamentally from superposition of paths.

Perfect communication occurs even when individual channels have zero capacity.

Abstract

Quantum mechanics is compatible with scenarios where the relative order between two events can be indefinite. Here we show that two independent instances of a noisy process can behave as a perfect quantum communication channel when used in a coherent superposition of two alternative orders. This phenomenon occurs even if the original process has zero capacity to transmit quantum information. In contrast, perfect quantum communication does not occur when the message is sent directly from the sender to the receiver through a superposition of alternative paths, with an independent noise process acting on each path. The possibility of perfect quantum communication through independent noisy channels highlights a fundamental difference between the superposition of orders in time and the superposition of paths in space.