  New tools and approaches of quantum optics offer a unique opportunity to
generate light pulses carrying a precise number of photons. Accurate control
over the light pulses helps to improve the characterization of photo-induced
processes. Here, we interface a specialized light source which provides flashes
containing just one photon, with retinal rod cells of Xenopus laevis toads. We
provide unambiguous proof of single photon sensitivity of rod cells without
relying on the statistical modeling. We determined their quantum efficiencies
without the use of any pre-calibrated detectors, and obtained the value of
29+-4.7 %. Our approach provides the path for future studies and applications
of quantum properties of light in phototransduction, vision, and
photosynthesis.
