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In the primary step of light-harvesting, the energy of a photon is capturedin antenna chlorophyll as an exciton. Its efficient conversion to storedchemical potential occurs in the special pair reaction center, which has to bereached by down-hill ultrafast excited state energy transport. The interactionbetween the chromophores leads to spatial delocalization and quantum coherenceeffects, the importance of which depends on the coupling between thechlorophylls in relation to the intensity of the fluctuations andreorganization dynamics of the protein matrix, or bath. The latter induceuncorrelated modulations of the site energies, and thus quantum decoherence,and localization of the exciton. Current consensus is that under physiologicalconditions quantum decoherence occurs on the 10 fs time scale, and quantumcoherence plays little role for the observed picosecond energy transferdynamics. In this work, we reaffirm this from a different point of view byfinding that the true onset of electronic quantum coherence only occurs atextremely low temperatures of ~20 K. We have directly determined the excitoncoherence times by two-dimensional electronic spectroscopy of theFenna-Matthew-Olson complex over an extensive temperature range with asupporting theoretical modelling. At 20 K, electronic coherences persist out to200 fs (close to the antenna) and marginally up to 500 fs at thereaction-center side. It decays markedly faster with modest increases intemperature to become irrelevant above 150 K. This temperature dependence alsoallows disentangling the previously reported long-lived beatings. We show thatthey result from mixing vibrational coherences in the electronic ground state...

Disentangling Dynamical Quantum Coherences in the Fenna-Matthews-Olson Complex