I see that the minimum number of photons detectable by humans has been measured currently as three, but the experiments are going further:
Gisin has pioneered experiments1 to see how the human eye responds to ‘quantum-weirdness’Fascinating...
effects. Although effects such as photons being in multiple places at once are well known, making humans part of the experiment “brings us closer to the quantum phenomenon”, he says.
Anthony Leggett, a Nobel Prize-winning theoretical physicist who is also at Urbana-Champaign and who inspired Holmes’s work, says that quantum weirdness should disappear somewhere between the scale of atoms and that of human bodies. “We don’t know at what stage it’s going to break down — or how." Holmes's study will probably validate standard interpretations of quantum physics, he says, which assume that a photon that is in a ‘superposition’ of two states will essentially choose one option when it comes into contact with a detector — whether that is an artificial photon counter or a rod cell.
But in principle, says physicist Angelo Bassi at the University of Trieste in Italy, each of the photon's personas could hit a rod cell, and that superposition could persist up to the brain. If so, there could even be “something like a superposition of two different perceptions, even if just for an instant”.