  Recently, exciton-photon strong coupling has been proposed as a means to
control and enhance energy transfer in ensembles of organic molecules. Here, we
demonstrate that the exciton dynamics in an archetypal purple bacterial
photosynthetic unit, composed of six LH2 antennas surrounding a single LH1
complex, is greatly modified by its interaction with an optical cavity. We
develop a Bloch-Redfield master equation approach that accounts for the
interplay between the B800 and B850 bacteriochlorophyll molecules within each
LH2 antenna, as well as their interactions with the central LH1 complex. Using
a realistic parametrization of both photosynthetic unit and optical cavity, we
investigate the formation of polaritons in the system, revealing that these can
be tuned to accelerate its exciton dynamics by three orders of magnitude. This
yields a significant occupation of the LH1 complex, the stage immediately prior
to the reaction center, with only a few-femtosecond delay after the initial
excitation of the LH2 B800 pigments. Our theoretical findings unveil
polaritonic phenomena as a promising route for the characterization, tailoring,
and optimization of light-harvesting mechanisms in natural and artificial
photosynthetic processes.
