Coherences in Molecular Excitation with Natural Incoherent Light?
2D photon echo studies on light harvesting systems
have generated considerable interest and controversy regarding
the possible role of quantum coherence effects in biological systems.
As we have previously shown, such studies rely on the response of
molecular systems to pulsed laser excitation, which is
dramatically different than the response to natural incoherent
light. Significantly, the latter produces mixed stationary states,
devoid of time dependent coherences. If this would be ``the whole
story", then the observed coherences are essentially irrelevant.
We will describe the origin of the above result and then discuss recent
developments in this area, including (a) the importance of various
decoherence time scales for reaching stationary states in natural incoherent
light, (b) the role of doorway states in the molecular response, and (c) the
significance of long lived coherences associated with Agarwal-Fano resonances.
Examples will be chosen from basic three level V-systems,
dynamics in large molecules, and Rydberg atoms
interacting with the cosmic microwave background. The significance of the
results for natural light harvesting systems will be emphasized.