Physicists Create New Form of Light

A research team led by scientists at MIT and Harvard has observed groups of three photons interacting and, in effect, sticking together to form a completely new kind of photonic matter. The behavior could open a path toward using photons in quantum computing, according to the team, or even more fanciful science fiction applications like—you guessed it—lightsabers.

“The interaction of individual photons has been a very long dream for decades,” says Vladan Vuletic, the Lester Wolfe Professor of Physics at MIT. Vuletic leads the MIT-Harvard Center for Ultracold Atoms together with Harvard’s Professor Mikhail Lukin.

The pair, joined by a team at the center, have been looking for ways, both theoretical and experimental, to encourage interactions between photons. But photons that make up light do not generally interact. Two flashlights set to cross beams in a dark room, for example, simply pass each other by instead of combining into one single, luminous stream. In controlled experiments, however, researchers from the center found that when they shone a very weak laser beam through a dense cloud of ultracold rubidium atoms, rather than exiting the cloud as single, randomly spaced photons, the photons bound together in pairs or triplets, suggesting some kind of interaction—in this case, attraction—taking place among them.

In 2013, the team had observed pairs of photons interacting and binding together for the first time, creating an entirely new state of matter. In its new work, published in the journal Science, the team pressed to see whether interactions could take place between not only two photons, but more. “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,” Vuletic says. “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 used the same experimental approach used to observe two-photon interactions, cooling a cloud of rubidium atoms to a standstill with ultracold temperatures, just a millionth of a degree above absolute zero. Through the cloud of immobilized atoms, the researchers then shine a very weak laser beam—so weak, in fact, that only a handful of photons travel through the cloud at any one time. The researchers then measure the photons as they come out the other side of the atom cloud. In the new experiment, the team found that the photons streamed out as pairs and triplets, rather than exiting the cloud at random intervals, as single photons having nothing to do with each other.

In addition to tracking the number and rate of photons, the team measured the phase of photons, before and after traveling through the atom cloud. A photon’s phase indicates its frequency of oscillation. “The phase tells you how strongly they’re interacting, and the larger the phase, the stronger they are bound together,” says Harvard’s Aditya Venkatramani, a scientist on the team. The group observed that as three-photon particles exited the atom cloud simultaneously, their phase was shifted compared to what it was when the photons didn’t interact at all, and was three times larger than the phase shift of two-photon molecules. “This means these photons are not just each of them independently interacting, but they’re all together interacting strongly.”

The researchers then developed a hypothesis to explain what might have caused the photons to interact in the first place, suggesting that as a single photon moves through the cloud of rubidium atoms, it briefly lands on a nearby atom before skipping to another atom, like a bee flitting between flowers, until it reaches the other end. If another photon is simultaneously traveling through the cloud, it can also spend some time on a rubidium atom, forming a polariton—a hybrid that is part photon, part atom. Then two polaritons can interact with each other via their atomic component. At the edge of the cloud, the atoms remain where they are, while the photons exit, still bound together.

The researchers found that this same phenomenon can occur with three photons, forming an even stronger bond than the interactions between two photons. “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 within the atom cloud occurs over a millionth of a second. And it is this interaction that triggers photons to remain bound together, even after they’ve left the cloud. “What’s neat about this is, when photons go through the medium, anything that happens in the medium, they ‘remember’ when they get out,” Cantu says.

This means that photons that have interacted with each other, in this case through an attraction between them, can be thought of as strongly correlated, or entangled – 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 will look for ways to coerce other interactions such as repulsion, where photons may scatter off each other like billiard balls. “It’s completely novel in the sense that we don’t even know sometimes qualitatively what to expect,” Vuletic says. “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.”