Single-photon absorption by single photosynthetic light-harvesting complexes. (arXiv:1801.04924v1 [physics.chem-ph])

We provide a unified theoretical approach to the quantum dynamics of
absorption of single photons and subsequent excitonic energy transfer in
photosynthetic light-harvesting complexes. Our analysis combines a continuous
mode <n>-photon quantum optical master equation for the chromophoric system
with the hierarchy of equations of motion describing excitonic dynamics in
presence of non-Markovian coupling to vibrations of the chromophores and
surrounding protein. We apply the approach to simulation of absorption of
single-photon coherent states by pigment-protein complexes containing between
one and seven chromophores, and compare with results obtained by excitation
using a thermal radiation field. We show that the values of excitation
probability obtained under single-photon absorption conditions can be
consistently related to bulk absorption cross-sections. Analysis of the
timescale and efficiency of single-photon absorption by light-harvesting
systems within this full quantum description of pigment-protein dynamics
coupled to a quantum radiation field reveals a non-trivial dependence of the
excitation probability and the excited state dynamics induced by exciton-phonon
coupling during and subsequent to the pulse, on the bandwidth of the incident
photon pulse. For bandwidths equal to the spectral bandwidth of Chlorophyll a,
our results yield an estimation of an average time of ~0.09 s for a single
chlorophyll chromophore to absorb the energy equivalent of one
(single-polarization) photon under irradiation by single-photon states at the
intensity of sunlight.

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