In which color human eye is most sensitive?
Answer to the additional question: "what is the quantum efficiency of the eye?" I remember old studies reporting that eight photons reaching the photoreceptors can be detected by a human observer as a flash of light in certain laboratory conditions. The more normal range would certainly be lower than 0.125 say 0.05 - 0.01. Cooling down laboratory animals thus reducing thermal noise in the retina enhances sensitivity but means longer integration times and lower spatial and temporal resolution.