The Role of Quantum Decoherence in FRET

Resonance energy transfer has become an indispensable experimental tool for single-molecule and single-cell biophysics. 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 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. Although J. R. Oppenheimer and T. F\"orster navigated these subtleties successfully, it remains hard to find an elementary derivation in modern language. The key step involves acknowledging quantum decoherence. Appreciating that aspect can be helpful when we attempt to extend our understanding to situations where F\"orster's original analysis is not applicable.