Human eyes can detect the smallest units of light

In some ways, the human eye is superior to the most sensitive light-sensing devices.

By Brooks Hays

NEW YORK, July 21 (UPI) -- New research suggest the human eye is remarkably sensitive to light. In a recent study, participants were able to detect the presence of a single photon inside a pitch black room.

"If you imagine this, it is remarkable: a photon, the smallest physical entity with quantum properties of which light consists, is interacting with a biological system consisting of billions of cells, all in a warm and wet environment," researcher Alipasha Vaziri, an associate professor and head of the Laboratory of Neurotechnology and Biophysics at Rockefeller University, said in a news release.


In some ways, the human eye is superior to the most sensitive light-sensing devices.

"The response that the photon generates survives all the way to the level of our awareness despite the ubiquitous background noise," said Vaziri. "Any man-made detector would need to be cooled and isolated from noise to behave the same way."

The single photon didn't always register in the eyes of the human study participants. Flashing a single photon a few seconds prior to a second single photon, increased the chances of the light being witnessed.

Previous attempts to measure the lower limits of human eyesight settled on a threshold of five to eight photons, but researchers say these experiments suffered from faulty equipment. Controlling the precise number of emitted photons is difficult and required a specialized device.


The device borrows from technology used in quantum optics. It's called a spontaneous parametric down-conversion, or SPDC. It emits two photons, one directed at the subject's eye and another at a detector.

Researchers had study participants choose between two brief intervals of time, one featuring a single photon emission and one without. Over the course of 30,000 trials, participants identified the single photon at a rate greater than chance.

The scientists published their findings in the journal Nature Communications.

"What we want to know next is how does a biological system achieve such sensitivity?" Vaziri said. "How does it achieve this in the presence of noise? Is the mechanism unique to vision or could it tell us something more general on how other systems could have evolved to detect weak signals in the presence of noise?"

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