What’s the fewest number of photons our eyes can detect?

The cells in our eyes responsible for vision respond to the packets of light energy known as photons. As early as the 1880s, experiments by the American physicist Samuel Langley found humans could detect bursts of light consisting of just a few thousand photons. But in 2016 a team led by Alipasha Vaziri at Rockefeller University in New York carried out the ultimate test, using a special device capable of firing just one photon into the eye at a time. Three volunteers sat in pitch blackness for 40 minutes, and then had to say whenever they believed they’d noticed something from the device. The results suggested that while single photons can’t be consciously seen, our eyes can still faintly sense their presence.