Environment-Assisted Quantum Walks in Energy Transfer of Photosynthetic Complexes

Department ofMec hanical Engineering ,Massac husettsInstitute Tec hnolo gy 77 A venue Cambridg e MA 02139(Dated: May 18, 2008)Ener gy transfer within photosynthetic systems can display quantum effects such as delocalized exc itations.Recently , direct evidence of long-li ved coherence has been experimentally demonstrated for the dynamics ofthe Fenna-Matthe ws-Olson (FMO) protein comple x [G. S. Engel et al., Nature 446, 782 (2007)]. Ho we ver,the rele vance of quantum dynamical processes to the exciton transfer efÞcienc y is to a large extent unkno wn.Here, we de velop a general theoretical frame w ork for studying the role of quantum interference effects in ener gytransfer dynamics of molecular arrays interacting with a thermal bath. T o this end, we generalize continuous-time quantum w alks to non-unitary and temperature-dependent dynamics in Liouville space deri ved from amicroscopic Hamiltonian. Dif ferent ph ysical effects of coherence and decoherence processes are explored via auni versal measure for the ener gy transfer efÞcienc y and its susceptibility . In parti cular ,we demonstrate that forthe FMO comple x an effecti ve interplay between free Hamiltonian and thermal suctuations in the en vironmentleads to a substantial increase in ener gy transfer efÞcienc y of about 25%.