  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.
