Your eyes may be more sensitive than you ever thought possible.
Your eyes may be more sensitive than you ever thought possible. In a study published Tuesday in Nature Communications, researchers report that our warm, wet, multicellular eyes have evolved such a high level of sensitivity that they can, on occasion, detect a single photon aimed at the retina.
Even the most sophisticated man-made devices require a cool, temperature-controlled environment to achieve the same feat.
A single photon is the smallest particle that light is made of, and it is extremely hard to see. “It’s not like a dim flash of light or anything like that,” said Alipasha Vaziri, a quantum physicist at Rockefeller University in New York City and the senior author on the paper. “It’s more a feeling of seeing something rather than really seeing it.” He described it, as being “at the threshold of imagination.”
“If you’ve ever looked at a faint star in the night sky and one second you see it but the next second you don’t — it’s kind of like that,” he said. Previous studies dating back to the 1940s have shown that the human eye can detect as few as five to seven photons. But whether a single photon would be detectable proved elusive.
Vaziri’s team used a process known as spontaneous parametric down-conversion (SPDC) in which a high-energy photon decays spontaneously in a crystal into two lower energetic photons. One of these photons is directed to a subject’s eye while the other is sent to a detector. If the detector notes a photon, then the researchers know the other half of the pair had to be sent to the observer.
Before the experiment began, each participant sat in a completely dark room for about 40 minutes to make sure his eyes were fully adapted to the dark. His head was held in place with a headrest and a bite bar to ensure that when the photon was released it would be directed at the most sensitive part of his retina.
When the subject felt ready he triggered the start of the experiment by pressing a button. Soon he would hear two acoustical signals separated by one second. Afterwards, he was asked to indicate which signal he thought was accompanied by a photon and which one was a blank. Finally, he had to rate how confident he was in his choice on a three point scale. Vaziri said the strategy of forcing the participants to make a choice — when they weren’t sure it was right — helped push the subjects to maximum sensitivity. “It was key for the results of the study,” he said.
The researchers report that the subjects were able to correctly determine when a photon had been fired 51.6 per cent of the time, which may not sound that impressive, but is statistically significant.
