  In the primary step of light-harvesting, the energy of a photon is captured
in antenna chlorophyll as an exciton. Its efficient conversion to stored
chemical potential occurs in the special pair reaction center, which has to be
reached by down-hill ultrafast excited state energy transport. The interaction
between the chromophores leads to spatial delocalization and quantum coherence
effects, the importance of which depends on the coupling between the
chlorophylls in relation to the intensity of the fluctuations and
reorganization dynamics of the protein matrix, or bath. The latter induce
uncorrelated modulations of the site energies, and thus quantum decoherence,
and localization of the exciton. Current consensus is that under physiological
conditions quantum decoherence occurs on the 10 fs time scale, and quantum
coherence plays little role for the observed picosecond energy transfer
dynamics. In this work, we reaffirm this from a different point of view by
finding that the true onset of electronic quantum coherence only occurs at
extremely low temperatures of ~20 K. We have directly determined the exciton
coherence times by two-dimensional electronic spectroscopy of the
Fenna-Matthew-Olson complex over an extensive temperature range with a
supporting theoretical modelling. At 20 K, electronic coherences persist out to
200 fs (close to the antenna) and marginally up to 500 fs at the
reaction-center side. It decays markedly faster with modest increases in
temperature to become irrelevant above 150 K. This temperature dependence also
allows disentangling the previously reported long-lived beatings. We show that
they result from mixing vibrational coherences in the electronic ground state
...
