The mysterious phenomenon known as quantum entanglement—where objects seemingly communicate at speeds faster than light to instantaneously influence one another, regardless of their distance apart—was famously dismissed by Einstein as "spooky action at a distance." New experiments could soon answer skeptics by enabling people to see entangled pulses of light with the naked eye.



Although Einstein rebelled against the notion of quantum entanglement, scientists have repeatedly proved that measuring one of an entangled pair of objects, such as a photon, immediately affects its counterpart no matter how great their separation—theoretically. The current record distance is 144 kilometers, between the Canary Islands of La Palma and Tenerife.



Photons make up light—and the fact that scientists regularly entangle these tiny packets of energy raised the possibility that humans might actually be able to observe this effect. Now experiments to shoot entangled photons at the human eye are under development, and should take place later this year. "It's fascinating that entanglement is something we could see with the naked eye—it brings us closer to this strange quantum phenomenon," notes researcher Nicolas Gisin, a quantum physicist at the University of Geneva in Switzerland



Entanglement is measured by creating entangled particles, sending them to different detectors, and seeing how quickly a measurement on one influences the other. The idea for this experiment is simply to replace the photon detectors with human vision. Human retinas are surprisingly sensitive, capable of being triggered by roughly seven photons. And although they only have an efficiency of about 7 percent (of every 100 photons that enter the pupil, only about seven go on to reach the retina) they have a dark count of virtually zero, meaning they generate few if any false positives.



"The eye can actually detect single photons, but the signals that light sends to the brain are suppressed unless there are about seven—otherwise you would see flashes of light all the time—even in complete darkness," explains quantum physicist Seth Lloyd of the Massachusetts Institute of Technology.



First, Gisin and his colleagues will entangle a pair of photons, and then amplify these signals by entangling each of these photons with another ensemble of, say, 100 photons. In the arrangement they are currently developing, one pulse of photons would then be sent at a person, whereas the other would be sent at a conventional photon detector to test what the volunteer saw, Gisin says. "Although there's no reason to have human eyes on both sides, the final experiments can involve that," he added. Gisin and his colleagues have posted their plans to arXiv.org and submitted the idea for more formal publication in the journal Physical Review A.



People should not jump to the conclusion that such experiments will entangle people with machines or other people on a quantum level, Gisin cautions. Rather, "people will see photons that were entangled with each other," he explains.



Although conventional photon detectors can already accomplish these results, "it's fascinating from a human point of view—I would always prefer to see a galaxy with my naked eye rather than download a picture from the Internet," says quantum physicist Sandu Popescu at the University of Bristol in England, who did not participate in this research. "Also, it's interesting that no one until now was motivated to consider this kind of detector—you normally want to use the best detector possible, not something like the human eye." Entanglement is usually thought of as a rather fragile phenomenon, he added, and such experiments could highlight that it is robust enough "as to survive a person detecting it—much stronger than people usually appreciate."



Lloyd, who also is not part of the Gisin team for these experiments, said that it "would be pretty neat if you can still detect entanglement despite the fact you're using the naked eye, which is an imperfect detector that is pretty lousy." He added: "It's really solid work, and when they do have an experiment with a human, I want to be next on line."