| Summary: | Photosynthetic antenna complexes harvest sunlight and efficiently transport energy to the reaction center where charge separation powers biochemical energy storage. Under low light conditions, the quantum efficiency of light harvesting complexes is almost unity. The high quantum efficiency of photosynthetic antenna complexes has attracted immense theoretical and experimental studies. It is understood that energy transfer is an incoherent process. However, it was hypothesized the energy transfer can start as a coherent process before it becomes completely stochastic in nature. Recent discovery of existence of long lived quantum coherence during energy transfer has sparked the discussion on the role of quantum coherence on the energy transfer efficiency. Early works assigned observed coherences to electronic states, and theoretical studies showed that electronic coherences could affect energy transfer efficiency--- by either enhancing or suppressing transfer. However, the nature of coherences has been fiercely debated as coherences only report the energy gap between the states that generate coherence signals. Recent works have suggested that either the coherences observed in photosynthetic antenna complexes arise from vibrational wave packets on the ground state or, alternatively, coherences arise from mixed electronic and vibrational states. Understanding origin of coherences is important for designing molecules for efficient light harvesting.
The work presented here enumerates various technical, experimental and theoretical advances made over the past few years to understand the nature of coherent energy transfer dynamics in photosynthetic antenna complexes. The experimental and theoretical advances in interpreting 3rd order spectroscopic signals made GRAPES a powerful spectrometer for performing 3rd order non-linear electronic spectroscopic. 3rd order non-linear spectroscopy provides insight into correlations responsible for energy transfer and long lived coherence. The ability to further control each light-matter interaction by altering the state of polarization of light has been used to decipher the nature of coherence during energy transfer. The control over light-matter interaction has been used to explore the chiral dynamics of photosynthetic antenna complexes.
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