The smallest unit of light is a single photon. Consumer cameras could never dream of detecting just one, but a recent study has shown that the human eye itself is capable of such a feat.
The Vaziri Lab team of Rockefeller University recently published the study in which they used some clever engineering to get around a tricky issue: in the past, scientists could create light sources that averaged one photon per firing, but might have fired up to three photons in an individual instance, with no way of knowing the true count. To alleviate this issue, the team used a spontaneous parametric down-conversion (SPDC), in which a single high-energy photon is fired through a crystal that breaks it into two lower energy photons. One of these photons is sent to the subject and one to a detector; thus, if the detector registers a photon, the other half of the pair must have hit the subject.
Surprisingly, in 30,767 trials with 2,420 single-photon hits, subjects were able to detect the photon 51.6% of the time overall and 60% of the time when they rated themselves as "highly confident" in having seen one, both of which are statistically significant rates (above any rate that random variation alone could have reasonably caused). Furthermore, having recently seen a photon made the subjects more sensitive to seeing another, a result likely due to evolution. I think it's very neat that our body's own optical instruments are so highly sensitive.
Lead image by Wikipedia user ROTFLOLEB, used under Creative Commons.
[via LA Times]