Five years ago, physicists from Harvard and MIT achieved a world first by forcing a pair of photons to interact with one another in ways that shouldn't seem possible.
What do you do when you've achieved such a lofty goal? You try to add a third photon, of course.
With all eyes on light as the future of computing, researchers are keen to discover new ways to manipulate photons.
By most accounts, the massless particles that make up the electromagnetic spectrum don't have a whole lot to do with one another.
We often smash atoms together in giant accelerators and search for new physics in the resulting carnage.
The same can't be said for photons. You can cross even the strongest of laser beams without risking so much as a gentle bump between two light particles.
For years physicists theorised there were conditions where this rule could be bent, and in 2013 they finally saw it in action.
"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," Harvard physicist Mikhail Lukin said at the time.
To do this, they passed a weak laser consisting of a few photons through a cloud of rubidium atoms chilled to near standstill.
Moving from atom to atom the light hands over some of its energy. Yet a strange thing happens when a nearby photon tries to do the same thing.
It's called a Rydberg blockade – effectively, neighbouring particles can't be excited to the same degree.
So as one photon buzzes an atom, a nearby photon with the same properties can't cause another atom to share the same level of excitement. So it sticks around, briefly forming an atom-light hybrid called a polariton.
As a result, there's a pushing and pulling of polaritons as the photons slowly make their way through the rubidium cloud. On exiting out the other side, they end up stuck together.
The same team of physicists has now used the same setup to determine if this special partnership could also be a triad, by throwing a third photon into the mix.
"For example, you can combine oxygen molecules to form O2 and O3 (ozone), but not O4, and for some molecules you can't form even a three-particle molecule," says the study's senior author, Vladan Vuletic from MIT.
"So it was an open question: Can you add more photons to a molecule to make bigger and bigger things?"
Sure enough, out popped clusters of photons in twos and threes, showing it was indeed possible. And sticking together these pairs and triplets of photons into a kind of "molecule" could have many useful applications.
All of this lays the groundwork for technologies that no longer use clunky old electrons to do the grunt work of computers, but photons that can be entangled, encoded, and sent long distances at high speeds packed with more information.
So what's next for the team? Will we be seeing photon quads? Vuletic is open minded.
"With repulsion of photons, can they be such that they form a regular pattern, like a crystal of light? Or will something else happen? It's very uncharted territory."
This research is published in Science.