We study responses of live retinal photoreceptors to light sources with different photon statistics. We show the ability of the cells to discriminate between the sources, down to the level of single photons.
We analyzed the electrophysiological response of an isolated rod photoreceptor of Xenopus laevis under stimulation by coherent and pseudothermal light sources. Using the suction-electrode technique for single cell recordings and a fiber optics setup for light delivery allowed measurements of the major statistical characteristics of the rod response. The results indicate differences in average responses of rod cells to coherent and pseudothermal light of the same intensity and also differences in signal-to-noise ratios and second-order intensity correlation functions. These findings should be relevant for interdisciplinary studies seeking applications of quantum optics in biology.
Our eyes are able to encode information from broad daylight down to only a handful of photons, but could we perceive a single photon? For decades the intrinsic Poisson variation of classical light sources has prevented definitive answers to this question. However, recent advances in quantum optics and non-classical light sources have reignited new interest in the question. Studies relying on sub-Poisson light sources have postulated that it is possible to produce sub-Poisson biological response distributions along the visual pathway, and even claimed that humans can perceive single photons. However, unambiguous experimental evidence for the prerequisite of sub-Poisson response statistics in the visual sciences has not been provided, that is, to which extent biological response statistics can differ between Poisson and sub-Poisson stimulation when taking retinal single-photon signal losses into account. In this thesis, we use a state-of-the-art sub-Poisson light source combined with the suction pipette technique to stimulate retinal rods in an axial geometry and measure their single-photon signal losses. We resolve the constraints set by the loss of single-photon signals to biological signal statistics from rods to human psychophysics. We show that due to the loss of single-photon signals it is impossible to distinguish the response distributions originating from Poisson or sub-Poisson light sources in the retina and at the level of perception on a biologically relevant timescale. Our results require fundamental re-evaluation of the previous results relying on sub-Poisson light stimulation in vision.