People can detect flashes of light as feeble as a single photon, an experiment has demonstrated—a finding that seems to conclude a 70-year quest to test the limits of human vision. In addition, Nicolas Gisin, a physicist at the University of Geneva in Switzerland, devised a new test to see if the human eye could pick out signs of ‘entanglement’1. This weird quantum effect inextricably links two or more objects in such a way that measurements carried out on one immediately change the properties of its partners, no matter how far apart they are. Quantum effects, such as entanglement, are usually confined to the invisible microscopic world and are detected only indirectly using precision instruments.
This is significant because it implies that we may be being impacted (“perception”) by things that we did not normally consider and that they could even involve quantum “weirdness.” One wonders if we could be moving towards explanations to various sensory anomalies.
The study, published in Nature Communications on July 19, “finally answers a long-standing question about whether humans can see single photons — they can!” says Paul Kwiat, a quantum optics researcher at the University of Illinois at Urbana–Champaign. The techniques used in the study also open up ways of testing how quantum properties—such as the ability of photons to be in two places at the same time—affect biology, he adds.
“The most amazing thing is that it’s not like seeing light. It’s almost a feeling, at the threshold of imagination,” says Alipasha Vaziri, a physicist at the Rockefeller University in New York City, who led the work and tried out the experience himself.
Experiments on cells from frogs have shown that sensitive light-detecting cells in vertebrate eyes, called rod cells, do fire in response to single photons. But, in part because the retina processes its information to reduce ‘noise’ from false alarms, researchers hadn’t been able to confirm whether the firing of one rod cell would trigger a signal that would be transmitted all the way to the brain. Nor was it clear whether people would be able to consciously sense such a signal if it did reach the brain. Experiments to test the limits of human vision have also had to wait for the arrival of quantum-optics technologies that can reliably produce one photon of light at a time.
In June 2015, physicist Rebecca Holmes, who works with Kwiat, reported evidence that humans can sense light flashes containing as few as three photons. (Those results are still unpublished, Kwiat says.) But that team had not yet tested the ultimate threshold of perception—the response to single photons.
Regarding entanglement, the study published in Nature shows that “The experiment is lovely because in this sense you can ‘see’ entanglement,” says [Fabio] Sciarrino [a scientist in the field]. “It brings quantumness closer to human experience.”