Quantum coherent dynamics in allophycocyanin trimer complex.

For years, scientists have debated whether plants and algae use quantum coherence to enhance the transfer of light energy during photosynthesis. Some studies have suggested that wavelike quantum effects might help energy flow more efficiently between pigments. To investigate this idea, we studied allophycocyanin (APC), a light-harvesting protein complex found in cyanobacteria. Using a combination of ultrafast laser spectroscopic methods and rigorous hierarchical equations-of-motion simulations, we examined both the trimer protein complex and its monomer subunit. The experiments detected rapid oscillations in the optical signals that resembled quantum beats. However, temperature-dependent studies and further analysis showed that these signals were caused by vibrations rather than true quantum superpositions. The excitonic transitions of APC exhibit ultrafast dephasing, within ∼40 femtoseconds at low temperatures and ∼20 femtoseconds at room temperature. This is much shorter than the time required for energy to move between chromophores, which is around 344 femtoseconds. In contrast, certain vibrational motions lasted much longer and remained largely unaffected by temperature. Similar vibrations were observed in both the full protein complex and its isolated subunits, confirming that long-lived oscillations do not rely on interactions between pigments. These findings suggest that long-lived oscillations in APC are predominantly vibrational rather than functionally significant electronic coherence.