Probing Photosynthetic Light Harvesting with a Single Photon

The process of converting light energy into chemical energy via photosynthesis sustains most of life on earth. However, fundamental questions remain open regarding the initial steps of photosynthesis, despite advances over the past several decades in understanding the dynamics of energy transfer in photosynthetic systems using ultrafast spectroscopies. Here, by developing a photon-counting quantum light spectroscopy that probes photosynthetic light harvesting with a single photon at a time, we experimentally demonstrate that photosynthesis begins and proceeds with a single quantum of energy [1]. We report the observation of individual single-photon absorption and emission events in spatially distinct regions in photosynthetic systems. We show that a complex material system can be prepared and studied with only a single excitation, without perturbation by other photons or averaged over many simultaneous excitations. The experiments were carried out on an ensemble of pigment-protein complexes light-harvesting 2 from purple bacteria <i>Rhodobacter sphaeroides</i> under ambient conditions in vitro. Our results advance a true microscopic mechanism of energy conversion in photosynthesis at a fundamental single quantum regime, beyond the semi-classical picture of spectroscopy, and open a new domain of experiments using quantum light to probe complex material systems.<br/><br/><br/>[1] Quanwei Li, Kaydren Orcutt, Robert L. Cook, Javier Sabines-Chesterking, Ashley L. Tong, Gabriela S. Schlau-Cohen, Xiang Zhang, Graham R. Fleming† & K. Birgitta Whaley†,<br/>Single-photon absorption and emission from a natural photosynthetic complex,<br/><b><i>Nature</i></b>, in press.