Quantum coherence in networks

TL;DR

This paper demonstrates a method to verify quantum coherence in network scenarios without changing measurement settings, showing that quantum correlations surpass classical ones in multi-party networks.

Contribution

It introduces a novel approach to verify quantum coherence in networks without inputs and derives inequalities distinguishing quantum from classical correlations.

Findings

Quantum coherence can be verified without input adjustments.

Classical and quantum correlations diverge increasingly with more parties.

Quantum states violate newly derived classical inequalities.

Abstract

From a quantum information perspective, verifying quantum coherence in a quantum experiment typically requires adjusting measurement settings or changing inputs. A paradigmatic example is that of a double-slit experiment, where observing the interference pattern on the screen in a series of experimental settings where one, the other, and both slits are open unambiguously proves quantum coherence. Here we show that this is not necessary by verifying quantum coherence in a network scenario without the need for inputs. We show that there exist probability distributions for joint outcomes of three parties in a triangular network with independent sources that cannot be replicated using classical resources. Furthermore, we generalize our results to $n$-party networks and show that the discrepancy between correlations in classical and quantum networks increases with the number of parties. To this end, we derive nonlinear inequalities that are satisfied by classical correlations and find quantum states that violate them.