The human eye is a remarkable photodetector, with a detection efficiency near unity for numbers of incident photons above a certain small threshold, while being blind to those below this level. This leads to the interesting possibility of doing quantum optics using the human eye as a detector. This would need to be based on a suitable quantum system, however, one with interesting quantum entanglement present but with several photons in whatever states were to be detected.
Pavel Sekatski and colleagues at the University of Geneva have shown that by using stimulated emission to amplify photons from correlated pairs it is possible to create multiphoton states that can robustly show violations of Bell's inequality, even with high losses. The multiphoton states essentially act as witnesses to the entangled states that they came from. While the actual experiment with a human eye might be a little tricky, this work does suggest interesting possibilities in quantum information technology that are far from obvious.