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Dynamic Coherence in Excitonic Molecular Complexes under Various Excitation Conditions

We investigate the relevance of dynamic quantum coherence in the energy transfer efficiency of molecular aggregates. We contrast the dynamics after excitation of a quantum mechanical system with that of a classical system. We demonstrate how a classical description of an ensemble average can be satisfactorily interpreted either as a single system driven by a continuous force or as an ensemble of systems each driven by an impulsive force. We derive the time evolution of the density matrix for an open quantum system excited by light or by a neighboring antenna. We argue that unlike in the classical case, the quantum description does not allow for a formal decomposition of the dynamics into sudden jumps in the quantum mechanical state. Rather, there is a natural finite time-scale associated with the excitation process. We propose a simple experiment how to test the influence of this time scale on the yield of photosynthesis. Because photosynthesis is intrinsically an average process, the efficiency of photosynthesis can be assessed from the quantum mechanical expectation value calculated from the second-order response theory, which has the same validity as the perturbative description of ultrafast experiments. We demonstrate using typical parameters of the currently most studied photosynthetic antenna, the Fenna-Matthews-Olson (FMO) complex, and a typical energy transfer rate from the chlorosome baseplate, that dynamic coherences are averaged out in the complex despite excitation proceeding through a coherent superposition of its eigenstates. The dynamic coherence averages out even when the FMO model is completely free of all dissipation and dephasing. We conclude that under natural excitation conditions coherent dynamics cannot be responsible for the remarkable efficiency of the photosynthesis even when considering the dynamics at a single molecular level.