Previously, photons were perceived as exceptionally individualistic particles. They never appeared to interact with each other, explaining why beams of light intersect and do not reflect with each other. Researchers at the Massachusetts Institute of Technology (MIT) asked: if photons are similar to atoms, can they be bound together into something like a molecule?
To answer this question, a team consisting of Ph.D. candidate in atomic physics at MIT Sergio Cantu; Aditya Venkatramani, a Ph.D. candidate in atomic physics at Harvard University, alongside several other collaborators, came up with a peculiar set-up: a cloud of chilled rubidium atoms, Smithsonian magazine reported.
Yes, rubidium — that alkali soft metal notorious for spontaneously igniting, particularly violently when coming in contact with water (explaining why it is kept either in airtight ampules or in kerosene).
The scientists evaporated rubidium with a laser, but kept the resulting cloud in an ultracold state. According to Smithsonian, this keeps the atoms diffused, moving slowly but in a highly-excited state, a decidedly abnormal condition from that found in nature.
After creating the strange substance, researchers again lit it with a laser, but so weakly that only a handful of photons entered the cloud. And here's where the real magic begins.
"Normally the photons would be traveling at the speed of light-or almost 300,000 kilometers per second. But after passing through the cloud, the photons creep along 100,000 times slower than normal," Smithsonian reported.
As mind-blowing as it may appear, the condition is plausible: the "slow light" phenomenon was noted in 2004 by researchers at UC Berkeley who slowed a beam of light to just 9.7 kilometers per second — almost 22,000 mph — still way below lightspeed, which, as everyone knows, is fixed at 670,615,200 miles per hour.
But the cherry on top of the MIT experimental cake came when researchers documented that the photons emerging from the rubidium cloud came out not as single particles, but as pairs, or, in some cases, triplets.
"These pairs and triplets also give off a different energy signature, a phase shift, that tells the researchers the photons are interacting," cited by Smithsonian magazine.
"We were not sure three photons would be a stable molecule or something we could even see," Venkatramani said. However, it turned out that groups of three photons were more stable than double groups. "The more you add, the more strongly they are bound," Venkatramani noted.
The team also found that the ordinarily weightless photons gained mass, albeit a fraction of an electron's mass, said a Daily Mail report, which termed the double-photon bond as a "new form of light."
Researchers now claim to have formulated a theoretical model for the process. According to this model, which remains to be proven, ordinary single photons interact with excited ultracold rubidium atoms and create a particle known as a polariton. Two polaritons can, and do, interact with each other. When these two interacting polaritons reach the edge of the cloud, rubidium atoms remain within the cloud, but photons tear away and go out — but as a pair or triplet structure.
Researchers note that there is still a lot of work to be done. Since the discovery has just been made, they do not know if they can make photons also repel each other. If proper tools are found, this would open a new world of photon manipulation, allowing for inventions and discoveries which are hard to imagine. Currently, researchers posit that this special form of light could be useful for quantum computing and the creation of uncrackable codes, ultra-precise clocks, unimaginably powerful computers and more.
In their more farout fantasies, researchers envision arranging photons into organized structures not unlike crystals, which are thought to be useful in quantum communication.