How to See Quantum Entanglement

Human eyes can detect the spooky phenomenon of quantum entanglement (What the hell is that?) — but only sometimes, a new study on the physics preprint website arXiv.org claims. While eyes can help determine if two individual photons were recently entangled, they can’t tell if the brighter bunch of photons that actually hit the retina are in this bizarre quantum state.

“In general you think these quantum phenomena that involve only a few particles, they’re really far removed from us. That is actually not so true anymore,” said physicist Nicolas Brunner of the University of Bristol. “You could really go to an experiment by just having people look at these photons, and from there really actually see entanglement.”

In an earlier paper, Brunner and colleagues at the University of Geneva in Switzerland sketched out an experiment in which a human observer could replace a standard quantum detector. This isn’t as far-fetched as it sounds, they say, because the eye’s most important job is to be a sensitive photon detector.

The researchers would first prepare two entangled photons — photons whose quantum properties are so intimately linked that one always knows what the other is doing. When an aspect of one photon’s quantum state is measured, the other photon changes in response, even when the two photons are separated by large distances.

The researchers would send one photon to a standard detector and the other to a human observer in a dark room. The human would see a dim point of light in either the right or left field of view, depending on the photon’s quantum state. If those flashes of light correlate strongly enough with the output of the ordinary photon detector, then the scientists can conclude that the photons are entangled.