Communicating via ignorance: Increasing communication capacity via superposition of order

TL;DR

This paper demonstrates that indefinite causal order allows for non-zero information transmission through noisy depolarising channels, surpassing classical limitations and opening new avenues in quantum communication.

Contribution

It introduces the concept of using superposition of channel order to enable communication through otherwise completely noisy channels, with experimental validation.

Findings

Achieved 0.049 bits communication with fully depolarising channels in theory.

Experimentally achieved 0.034 bits in the fully depolarising case.

Achieved 0.64 bits with a depolarising and a known unitary channel.

Abstract

Classically, no information can be transmitted through a depolarising, that is a completely noisy, channel. We show that by combining a depolarising channel with another channel in an indefinite causal order---that is, when there is superposition of the order that these two channels were applied---it becomes possible to transmit significant information. We consider two limiting cases. When both channels are fully-depolarising, the ideal limit is communication of 0.049 bits; experimentally we achieve $(3.4{\pm}0.2){\times}10^{-2}$ bits. When one channel is fully-depolarising, and the other is a known unitary, the ideal limit is communication of 1 bit. We experimentally achieve 0.64${\pm}$0.02 bits. Our results offer intriguing possibilities for future communication strategies beyond conventional quantum Shannon theory.