Light is made of individual photons, and these particles do not usually interact with each other. But a team of physicists from MIT and Harvard have successfully made three photons interact to form a completely new kind of photonic matter, in a breakthrough that could open a path toward using photons in quantum computing.

Professor Vladan Vuletic of MIT and Mikhail Lukin of Harvard University lead the MIT-Harvard Center for Ultracold Atoms, and together they have spent years looking for ways to make photons interact with each other. Their first success came in 2013 when they observed pairs of photon photons interacting and binding together, creating an entirely new state of matter – but they weren’t sure if interactions could take place between not only two photons, but more.

“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 Vuletic in a news release. “So it was an open question: Can you add more photons to a molecule to make bigger and bigger things?”

To find out, the team created the same experimental set up they used to observe two photons interactions. The experiments involved shining a very weak laser beam through the cloud of rubidium atoms chilled to a millionth of a degree above absolute zero. The reason they cooled rubidium atoms was to slow them down or stop them. Their next step was to shine a very weak laser beam through this cloud of supercooled atoms, so that they could measure few of the photons that managed to travel through the cloud.

Normally, photons have no mass, and they travel at the speed of light – 300,000 kilometers per second. But researchers found that the photons that came out off the other side of the cloud were strongly bound together with each other and they streamed out as pairs and triplets as “single photons.” And what’s more? The bound photons had actually acquired a fraction of the mass of an electron, and moved 100,000 times slower than the normal photon.

The experiment did not end there.

The team also developed a hypothesis to explain what caused the photons to interact in the first place. Their model suggests that as a single photon moves through the cloud of rubidium atoms, it basically lands on an atom that’s at close quarter and then jumps on to the next.

Another photon travelling through the cloud also briefly binds to a rubidium atom, forming a polariton — a hybrid that is part photon, part atom. If multiple polaritons formed in the cloud, they interact with other by the way of their atomic components. And when they reach the edge of the cloud, the rubidium atoms stay behind, while the photons exit, still bound together.

The same phenomenon occurs with three photons – and they form an even stronger bond than when they make two photons interact.

“What was interesting was that these triplets formed at all,” Vuletic says. “It was also not known whether they would be equally, less, or more strongly bound compared with photon pairs.”

The entire interaction process occurs within a millionth of a second, and researchers say photons that have interacted with each other can be considered as “entangled,” which is a key property for any quantum computing bit.

“Photons can travel very fast over long distances, and people have been using light to transmit information, such as in optical fibers,” Vuletic says. “If photons can influence one another, then if you can entangle these photons, and we’ve done that, you can use them to distribute quantum information in an interesting and useful way.”

Going forward, the team aims to figure out ways to coerce other interactions among photons, such as repulsion – where photons scatter off each other.

“It’s completely novel in the sense that we don’t even know sometimes qualitatively what to expect,” explains Vuletic. “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.”

The study, entitled “Observation of three-photon bound states in a quantum nonlinear medium” has been published in the journal Science.