People can detect a flash of light as minute as a single photon, Scientific American reports. In the experiment—published in Nature Communications—participants would hear two sounds after pushing a button, some of the sounds accompanied by a photon, others not.
When asked when they thought they saw a photon, the participants were able to answer correctly more frequently than if they were guessing at random. Some researchers are unsure how conclusive the experiment is, however, because all of the participants were male, and men and women have different physiologies.
Source - sciecnemag.org