Quantum coherence explored at the level of individual light-harvesting complexes

Quantum mechanical effects in biological processes, such as natural photosynthesis, are intriguing and lively debated issues. The initial steps of photosynthesis comprise the absorption of sunlight by pigmentprotein complexes as well as rapid and remarkably efficient funnelling of excitation energy to a reaction centre. In these energy transfer processes oscillatory signatures of surprisingly long-lived coherences have been found by 2-dimensional spectroscopy on ensembles of various light-harvesting complexes. These data have been modelled in terms of environmentally assisted quantum transport with a careful balance between coherence, dissipation, and dephasing. This precarious equilibrium is influenced by temporal, spatial and spectral inter-complex variations on a nanoscopic level, caused by the highly dynamic environments and broad conformational diversity in functioning bio-systems. Unfortunately, ensemble experiments fail to resolve this. Hence, to unravel the nature of energy transfer in light-harvesting and to uncover the possible biological role of long-lived quantum coherences in the energy transfer dynamics, it is crucial to probe the ultrafast response of antenna proteins beyond the ensemble average and to test the robustness of coherences against perturbations on the level of individual complexes. Here we demonstrate ultrafast quantum coherent energy transfer within single light-harvesting complexes of a purple bacterium under physiological conditions. We find that quantum coherences between electronically coupled energy eigenstates persist at least 400 fs, significantly longer than previously reported, and that distinct energy transfer pathways can be identified in each complex. Strikingly, also changing transfer pathways in individual complexes on time scales of seconds are revealed. This is attributed to structural rearrangements of the pigment molecules and the surrounding protein scaffold caused by ubiquitous thermal disorder at elevated temperatur- s. Our data indicate that long-lived quantum coherence indeed plays a biological role as it renders energy transfer robust in the presence of disorder.