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
photoinduced processes. Here, we study interaction of a specialized light source which provides flashes
containing just one photon, with retinal rod cells of Xenopus laevis toads. We provide unambiguous proof
of the single-photon sensitivity of rod cells without relying on the statistical modeling. We determine their
quantum efficiencies without the use of any precalibrated detectors and obtain the value of ð29 4.7Þ%.
Our approach provides the path for future studies and applications of quantum properties of light in
phototransduction, vision, and photosynthesis.