Entangled photons might not have to originate from the same location

Finding challenges our fundamental understanding of quantum theory.

Do entangled photons have to begin from the same place? Maybe not.
(Photo: marc/Flickr)

Entanglement is one of the spookiest discoveries of quantum theory. (In fact, Einstein himself referred to the phenomenon as "spooky action at a distance.") It involves two particles, such as photons, that are somehow instantaneously linked to one another, even if they are separated by vast distances.

The phenomenon is super anti-intuitive, but there's at least one aspect to entanglement that our intuitions can hang their hats on: entangled particles are at least supposed to originate from the same location.

Or maybe not.

Researchers at the University of East Anglia (UEA) have identified entangled photons that each emerged from different points in space. It's a profound discovery that makes the phenomenon of entanglement spookier than ever, and it could challenge the very foundation of quantum theory itself.

"Until now, it has been assumed that such paired photons come from the same location," explained UEA's David Andrews, in a university news release. "Now, the identification of a new delocalized mechanism shows that each photon pair can be emitted from spatially separated points, introducing a new positional uncertainty of a fundamental quantum origin."

The bizarre photons were observed while the UEA team was researching a process called spontaneous parametric down-conversion (SPDC), which involves firing photons through a crystal to generate entangled pairs. Basically, it works like this: a single beam of light (usually blue in color) is fired through a specialized crystal that splits the light into two distinct red beams. When a single blue photon splits into two red ones, they can emerge as entangled.

What the UEA team observed, however, was entangled photons emerging from different points in the crystal, and that's not supposed to be how it works. The findings could mean that there are undefined limits to spatial resolution.

"Everything has a certain quantum 'fuzziness' to it, and photons are not the hard little bullets of light that are popularly imagined," suggested Andrews.

Theorists will need to mull over these results before anything definitive can be said about what it all means. But it goes to show that for all its mysteries, quantum theory still has a few tricks up its sleeve.