This article says it's barely perceptible to the naked eye: https://www.livescience.com/61763-single-atom-ion-trap-photo.html https://www.livescience.com/61763-

This article says it's barely perceptible to the naked eye: https://www.livescience.com/61763-single-atom-ion-trap-photo.html https://www.livescience.com/61763-single-atom-ion-trap-photo... People can detect (though not very reliably) single photons hitting a photoreceptor or ~7 of them entering the eye once they're totally dark-adapted, so that almost has to be true.