  Photosynthetic antenna complexes are responsible for absorbing energy from
sunlight and transmitting it to remote locations where it can be stored. Recent
experiments have found that this process involves long-lived quantum coherence
between pigment molecules, called chromophores, which make up these complexes.
Expected to decay within 100 fs at room temperature, these coherences were
instead found to persist for picosecond time scales, despite having no apparent
isolation from the thermal environment of the cell. This paper derives a
quantum master equation which describes the coherent evolution of a system in
strong contact with a thermal environment. Conditions necessary for long
coherence lifetimes are identified, and the role of coherence in efficient
energy transport is illuminated. Static spectra and exciton transfer rates for
the PE545 complex of the cryptophyte algae CS24 are calculated and shown to
have good agreement with experiment.
