Environment-Assisted Quantum Coherence in Photosynthetic Complex.

Recent experiments [ Engel et al. Nature, 2007, 446, 782-786 ] revealed the existence of surprisingly long-lived quantum coherence in the noisy biological environment of the photosynthetic Fenna-Matthews-Olson (FMO) complex. Such coherence can clearly play an important role in facilitating efficient energy transfer. The occurrence of quantum coherence in quantum transport is also implicated in excitation transport processes in conjugated polymers [ Collini et al. Science, 2009, 323, 369-373 ]. Even though these systems are strongly correlated, most theoretical studies invoke a Markovian approximation where the temporal correlation of bath fluctuations is neglected. We use an elegant nonperturbative method based on Kubo's quantum stochastic Liouville equation (QSLE) to study the effects of correlated non-Markovian bath fluctuations in several different limits and find the interesting result that fluctuations not only destroy coherence but under appropriate conditions can also facilitate it. We show that temperature has the most pronounced effect in the intermediate coupling limit where it can promote transition from coherent to incoherent transfer.