Our eyes can see single specks of light

For decades, researchers have wondered just how little light the eye can see. They now appear to have the answer. And it’s surprising. Our eyes can detect a single speck — what scientists call a photon or light particle, a new study suggests. If confirmed, this may allow scientists to use the human eye to test some basic features of physics on the super-small scale.

The new study also showed that the human eye detects single photons better when it has just seen another photon. This was “an unexpected phenomenon,” says Alipasha Vaziri. He is a physicist at Rockefeller University in New York City. Physicists study the nature and properties of matter and energy. Vaziri and colleagues described the results of their study July 19 in Nature Communications.

Earlier experiments indicated that people can see blips of light made up of just a few photons. But there had been no surefire way to tell if the eye registers single photons. That’s because single photons are hard to produce reliably. But Vaziri and his co-workers were able to do it.

They used a technique with a long name: spontaneous parametric down-conversion, or SPDC. Scientists send a high-energy photon into a crystal. Once inside, the single photon turns into two low-energy photons. One of those new photons is diverted to someone’s eye. The SPDC system deflects the photon to a detector. That detector confirms each photon that is produced.

During the experiment, people watch for the very dim flash of a photon. The participants also listen for warning beeps. And there will be two. One of them accompanies the photon. The other does not. The participant does not know which beep corresponds to a light speck. Each viewer reports which beep they thought announced a photon, and how confident they were that they were right.

The scientists ran the experiment 2,420 times. Participants guessed the correct beep slightly more times than should occur just due to chance. That seemingly unimpressive success rate had been expected. The reason: Most photons will not make it all the way through to the retina at the back of the eye. That’s the part of eye sensitive to light. When the retina picks up photons, it will alert the brain, which can then form a visual image. That means that in most trials, a participant wouldn’t be able to see a photon associated with either beep.

But in trials where the participants indicated they were most certain of their choice, they were correct 60 percent of the time. Such a success rate would be unlikely if humans were unable to see a single photon. The chance of such a fluke would be one in 1,000.

“It’s not surprising that the correctness of the result might rely on the [viewer’s] confidence,” says Paul Kwiat. He is a physicist at the University of Illinois at Urbana-Champaign who was not involved with the research. Those trials where participants were more confident may represent the times when photons succeeded in making it through to their retinas, he suggests.

The data also indicate that single photons may be able to prepare the brain to detect more dim flashes that follow. Participants were more likely to correctly identify a photon if they had been sent one less than 10 seconds earlier.

The eye as a physics tool

Scientists hope to use the SPDC photon-scouting technique to test whether humans can directly observe quantum weirdness.

Quantum mechanics is a field of physics that deals with the way matter behaves on the scale of atoms or their even tinier building blocks. And that’s the arena our eyes may be able to help scientists understand.

All scientists agree that by normal standards, the quantum world is weird. For instance, photons can be in two places at once. Scientists describe this state as the photons being in a quantum superposition.

Some physicists wonder whether it might be possible to send such quantum states to someone’s eye. If humans could directly observe the strange quantum behavior, rather than having to use other fancy detectors, they might be able to better understand it.

But Leonid Krivitsky isn’t convinced. He is a physicist at the Agency for Science, Technology and Research in Singapore. He claims to be “pretty skeptical about this idea of observing quantumness in the brain.” By the time the brain realizes an eye has seen a flash of light, those photon signals will have lost any potential quantum properties, he suspects.

If true, why should anyone care if the eye can see a single photon? Vaziri says it may point to understanding our pre-electric-light society. Imagine, he says, that “you are somewhere outside of a city in nature, and on a moonless night.” You will have only stars by which to navigate. And on average, he says, the number of photons that get into your eye will be “approaching the single photon regime.”

Having eyes sensitive enough to see single photons, he says, may have had some evolutionary advantage. It may have helped us adapt to a night-time world that existed before the advent of light bulbs.