Resonance energy transfer has become an indispensable experimental tool for single-molecule and single-cell biophysics, and a conceptual tool to understand bioluminescence and photosynthesis. Its physical underpinnings, however, are subtle: It involves a discrete jump of excitation from one molecule to another, and so we regard it as a strongly quantum-mechanical process. And yet its first-order kinetics differ from what many of us were taught about two-state quantum systems; quantum superpositions of the states do not seem to arise; and so on. The key step involves acknowledging quantum decoherence.
Ref: P C Nelson, Biophys J in press (2018).