Human vision can detect single photons with a probability above chance, reports a study published online this week in Nature Communications. This result provides new insights into the detection limits of the human eye.

Studies in the 1940s have established that human subjects are capable of reporting light signals as low as a few (five to seven) photons. However, whether humans can perceive a single photon has remained an open question, partly because of experimental constraints connected to the light source used to generate photons in previous experiments.

Alipasha Vaziri and colleagues designed a single-photon light source using quantum-optic technologies and test the detection limits of human vision in three participants. The light source system is capable of generating a correlated pair of photons: one photon was sent to the participant’s eye, and the other towards a highly-sensitive camera. During each trial, participants were presented with two light stimuli, one of which contained a single photon and the other was blank (in other words, it contained no light). Participants were asked to state which of the two stimuli contained a light signal. Based on the performance of the individuals across a total of 30,767 trials, the authors found that the averaged probability of the individuals correctly identifying a small flash of light containing a single photon from a blank is above chance.

The authors note that the retinal and brain-circuitry mechanisms underlying their findings are the scope of future work.