Seeing a single photon

Scientists have measured the response of retinal cells to single photons, proving unambiguously that the eye can detect individual photons.

EXPERIMENTS have shown that retinal cells are remarkably efficient detectors, able to respond even to very few photons. But these studies have been carried out with attenuated classical light sources, which emit a number of photons with a certain probability distribution. Hence, there are doubts on the exact number of photons stimulating the retina, and its quantum efficiency has to be inferred through statistical analysis.

Exploiting recent advances in single-photon sources driven by quantum communication applications, researchers in Singapore have now carried out the first experiments in which the retina was stimulated by a single photon. The results, which were published in a recent issue of Physical Review Letters, provide unambiguous proof that rod cells in the retina are sensitive to single photons and allow a precise characterisation of their quantum efficiency.