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We investigate the relevance of dynamic quantum coherence in the energytransfer efficiency of molecular aggregates. We contrast the dynamics afterexcitation of a quantum mechanical system with that of a classical system. Wedemonstrate how a classical description of an ensemble average can besatisfactorily interpreted either as a single system driven by a continuousforce or as an ensemble of systems each driven by an impulsive force. We derivethe time evolution of the density matrix for an open quantum system excited bylight or by a neighboring antenna. We argue that unlike in the classical case,the quantum description does not allow for a formal decomposition of thedynamics into sudden jumps in the quantum mechanical state. Rather, there is anatural finite time-scale associated with the excitation process. We propose asimple experiment how to test the influence of this time scale on the yield ofphotosynthesis. Because photosynthesis is intrinsically an average process, theefficiency of photosynthesis can be assessed from the quantum mechanicalexpectation value calculated from the second-order response theory, which hasthe same validity as the perturbative description of ultrafast experiments. Wedemonstrate using typical parameters of the currently most studiedphotosynthetic antenna, the Fenna-Matthews-Olson (FMO) complex, and a typicalenergy transfer rate from the chlorosome baseplate, that dynamic coherences areaveraged out in the complex despite excitation proceeding through a coherentsuperposition of its eigenstates. The dynamic coherence averages out even whenthe FMO model is completely free of all dissipation and dephasing. We concludethat under natural excitation conditions coherent dynamics cannot beresponsible for the remarkable efficiency of the photosynthesis even whenconsidering the dynamics at a single molecular level.

Dynamic Coherence in Excitonic Molecular Complexes under Various Excitation Conditions