Incoherent witnessing of quantum coherence

TL;DR

This paper investigates the challenges and paradoxes in detecting quantum coherence in complex systems, revealing conditions for valid claims and proposing a protocol using entangled probes for accurate detection.

Contribution

It identifies counterintuitive phenomena in coherence detection and introduces a protocol with entangled probes to reliably detect quantum coherence in inaccessible systems.

Findings

Detection of initial coherence can occur even with fully incoherent states.

Necessary and sufficient conditions for valid coherence claims are established.

Entangled probes can effectively detect quantum coherence under these conditions.

Abstract

Theoretical and experimental studies have suggested the relevance of quantum coherence to the performance of photovoltaic and light-harvesting complex molecular systems. However, there are ambiguities regarding the validity of statements we can make about the coherence in such systems. Here we analyze the general procedure for coherence detection in quantum systems and show the counterintuitive phenomenon of detecting a quantum system's initial coherence when both the input and output probe states are completely incoherent. Our analysis yields the necessary and sufficient conditions for valid claims regarding the coherence of directly inaccessible systems. We further provide a proof-of-principle protocol that uses entangled probes to detect quantum coherence satisfying these conditions, and discuss its potency for detecting coherence.