Communicating via ignorance: Increasing communication capacity via superposition of order

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.

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