  In this paper, we present a theoretical description to the quantum coherence
and decoherence phenomena of energy transfer in photosynthesis observed in a
recent experiment [see Science 316, 1462 (2007)]. As a successive two-color
laser pulses with selected frequencies cast on a sample of the photosynthetic
purple bacterium Rb. sphaeroides two resonant excitations of electrons in
chromophores can be generated. However, this effective two-level subsystem will
interact with its protein environment and decoherence is inevitable. We
describe this subsystem coupled with its environment as a dynamical spin-boson
model. The non-Markovian decoherence dynamics is described using a
quasi-adiabatic propagator path integral (QUAPI) approach. With the
photon-induced effective time-dependent level splitting energy and level flip
coupling coefficient between the two excited states and the environment-induced
non-Markovian decoherence dynamics, our theoretical result is in good agreement
with the experimental data.
