Perfect discrimination of no-signalling channels via quantum superposition of causal structures

TL;DR

This paper demonstrates that quantum superpositions of causal structures enable perfect discrimination of no-signalling channels, surpassing the capabilities of classical or fixed causal order circuits.

Contribution

It introduces a method using quantum superpositions of causal orders to perfectly distinguish no-signalling channels, a feat impossible with traditional circuits.

Findings

Quantum superpositions enable perfect discrimination of no-signalling channels.

Classical circuits cannot achieve perfect discrimination of these channels.

The approach advances understanding of quantum causal structures.

Abstract

A no-signalling channel transforming quantum systems in Alice's and Bob's laboratories is compatible with two different causal structures: (A < B) Alice's output causally precedes Bob's input and (B< A) Bob's output causally precedes Alice's input. I show that a quantum superposition of circuits operating within these two causal structures enables the perfect discrimination between no-signalling channels that can not be perfectly distinguished by any ordinary circuit.