Researchers have managed to coax photons into binding together to form molecules - a state of matter that, until recently, had been purely theoretical.
The discovery led by Harvard Professor of Physics Mikhail Lukin and MIT Professor of Physics Vladan Vuletic runs contrary to decades of accepted wisdom about the nature of light, researchers said.
Photons have long been described as massless particles which don't interact with each other - shine two laser beams at each other, he said, and they simply pass through one another.
"Most of the properties of light we know about originate from the fact that photons are massless, and that they do not interact with each other," Lukin said.
"What we have done is create a special type of medium in which photons interact with each other so strongly that they begin to act as though they have mass, and they bind together to form molecules. This type of photonic bound state has been discussed theoretically for quite a while, but until now it hadn't been observed," he said.
"When these photons interact with each other, they're pushing against and deflect each other. The physics of what's happening in these molecules is similar to what we see in the movies, said Lukin.
To get the normally-massless photons to bind to each other, Lukin and colleagues couldn't rely on something like the Force - they instead turned to a set of more extreme conditions.
Researchers began by pumping rubidium atoms into a vacuum chamber, then used lasers to cool the cloud of atoms to just a few degrees above absolute zero.
As the photons enter the cloud of cold atoms, Lukin said, its energy excites atoms along its path, causing the photon to slow dramatically.
As the photon moves through the cloud, that energy is handed off from atom to atom, and eventually exits the cloud with the photon, researchers said.
When Lukin and colleagues fired two photons into the cloud, they were surprised to see them exit together, as a single molecule.
The study was published in the journal Nature.