Quantum coherence in the photosynthesis apparatus of living cyanobacteria

Life as we know it cannot exist without photosynthesis, and even though the main photosynthetic mechanisms have been well investigated, some aspects are still unresolved. One example is the energy transfer to the reaction centers by accessory photosynthetic pigments after the absorption of photons. This process has an extremely high efficiency, which cannot be explained by a classical Foerster resonance energy transfer. However, a quantum mechanical process based on a coherent or wave-like energy transfer may provide an explanation for the high efficiency. In order to determine whether nature makes use of such a coherent process, we influence the potential coherence of photosynthetic pigments in vivo using an optical microresonator, which consists of two parallel silver mirrors separated only by the distance of a few wavelengths. The electromagnetic field inside such a microcavity is strongly confined, enabling coherent light-matter coupling. Here, we embedded living cyanobacteria of the species Synechococcus elongatus (strain PCC 7492) into the microresonator and exposed them to the confined electromagnetic field. The observation of vacuum Rabi splitting and anti-crossing observed in the transmission- and fluorescence spectra provides evidence of coherent coupling of the pigments with the resonator modes without harming the bacteria. Furthermore, we showed that not only some photosynthesis pigments are involved in this coupling, but all pigments in the excitation focus are coupled coherently. Our findings shed light on the function of quantum coherence in the evolution of photosynthetic organisms.