When you throw certain elements together like hydrogen or oxygen, they can bond in pairs or even triplets, forming O2 (oxygen) or O3 (ozone), for instance. Shine two flashlights together, however and ... crickets. The photons simply pass through each other like phantoms and there's no reaction whatsoever. That's because they have no mass or charge, though they can become highly energized in the form of X-rays or gamma rays.
To get the photons together, then, the researchers beamed a very weak laser through a dense cloud of rubidium atoms cooled to a hair above absolute zero. Instead of exiting randomly one at a time as you'd expect, they bound together in pairs or triplets, creating some form of entanglement. In addition, the normally mass-less photons gained some weight, as well -- a fraction of an electron's mass, but it's something.
The heftier photon "molecules," if you like, were considerably less nimble, too. Rather than moving at their regular 186,000 mile per second pace, they were moving 100,000 times slower, less than SpaceX's Falcon Heavy Tesla Roadster is at the moment, by my calculation.
How did this happen? When passing through the listless rubidium atoms, the photons passed on some of their energy. However, because of something called the Rydberg blockade, adjacent atoms can't be excited as much, and the less-agitated atom and a photon formed a hybrid called a "polariton." As the photons skipped between polaritons, they interacted with each other in ways they normally wouldn't, and some were still stuck together when they exited the cloud. This happenned slowly on a quantum scale, about a millionth of a second from entry to exit.
It's not the first time scientists have got photons together; some of the same team managed to get pairs of photons hooked together in 2013. The new discovery, however, marks the first time that three photons have been forced to interact.
The research is not just interesting to particle physicists, but it could eventually pave the way to new types of quantum computers used to crack cryptographic codes and solve difficult equations. The photon triplets are essentially entangled, so they could be used in "qubit" processors or for transmitting information over long distances. Having multiple photons entangled would allow for more robust, powerful systems. "The interaction of individual photons has been a very long dream for decades," said lead author and MIT professor Vladan Vuletic.