Human Eyes Can Detect Single Photon

The sensitivity of our vision pushes the absolute physical limits of detection. Deep within the retina, millions of specialized photoreceptors called rod cells are responsible for our ability to see in low-light conditions. These cells are so exquisitely fine-tuned that they can be activated by the energy from a single photon, the smallest possible particle of light. For this to happen, however, conditions must be perfect. The observer must be in a state of complete dark adaptation, a process that can take over 30 minutes, allowing the rod cells to reach their peak sensitivity.

While a single rod cell can detect a photon, this doesn't mean we consciously "see" it. The brain is a masterful signal processor, constantly filtering out random noise to create a stable picture of the world. A single cell firing could be a fluke, so the brain's visual cortex requires a near-simultaneous signal from a small cluster of rod cells before it will register the event as a genuine flash of light. This concept was first explored in a landmark 1942 experiment that estimated the threshold was around five to seven photons. The 2016 research, however, used advanced technology to finally confirm that while the eye's hardware can detect a single photon, our brain's software demands confirmation.