Vibronic resonances facilitate excited state coherence in light harvesting proteins at room temperature

TL;DR

This study demonstrates that vibronic resonances enable long-lived excited state coherence in light-harvesting proteins at room temperature, revealing quantum effects' role in photosynthesis.

Contribution

It provides the first direct experimental evidence linking vibronic mixing to sustained excited state coherence in photosynthetic complexes.

Findings

Identification of excited state coherent superpositions

Observation of low energy oscillations due to decoherence

Evidence of strong vibronic mixing among excited states

Abstract

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.