Coherences in Molecular Excitation with Natural Incoherent Light?

Abstract:

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.