  Until recently it was believed that photosynthesis, a fundamental process for
life on earth, could be fully understood with semi-classical models. However,
puzzling quantum phenomena have been observed in several photosynthetic
pigment-protein complexes, prompting questions regarding the nature and role of
these effects. Recent attention has focused on discrete vibrational modes that
are resonant or quasi-resonant with excitonic energy splittings and strongly
coupled to these excitonic states. Here we unambiguously identify excited state
coherent superpositions in photosynthetic light-harvesting complexes using a
new experimental approach. Decoherence on the timescale of the excited state
lifetime allows low energy (56 cm-1) oscillations on the signal intensity to be
observed. In conjunction with an appropriate model, these oscillations provide
clear and direct experimental evidence that the persistent coherences observed
require strong vibronic mixing among excited states.
