It's a glimpse of science fiction made fact: Scientists have created a new form of light that could someday be used to build light crystals. But before would-be Jedis start demanding their sabers, the advance is far more likely to lead to intriguing new ways of communicating and computing, researchers report this week in Science.
Light is made up of photons—speedy, tiny packets of energy. Typically, photons do not interact with each other at all, which is why when using flashlights "you don't see the light beams bounce off each other, you see them go through each other," explains Sergio Cantu, a Ph.D. candidate in atomic physics at the Massachusetts Institute of Technology. In new experiments, however, the physicists coaxed individual photons to cozy up to each other and link, similar to the way individual atoms stick together in molecules.
The photon dance happens in a lab at MIT where the physicists run table-top experiments with lasers. Cantu, his colleague Aditya Venkatramani, a Ph.D. candidate in atomic physics at Harvard University, and their collaborators start by creating a cloud of chilled rubidium atoms. Rubidium is an alkali metal so it typically looks like a silver-white solid. But vaporizing rubidium with a laser and keeping it ultracold creates a cloud the researchers contain in a small tube and magnetize. This keeps the rubidium atoms diffuse, slow moving and in a highly excited state.
Then the team fires a weak laser at the cloud. The laser is so weak that just a handful of photons enter the cloud, a press release from MIT explains. The physicists measure the photons when they exit the other side of the cloud and that is when things get weird.
Normally the photons would be traveling at the speed of light—or almost 300,000 kilometers per second. But while passing through the cloud, the photons creep along 100,000 times slower than normal. Also, instead of exiting the cloud randomly, the photons come through in pairs or triplets. These pairs and triplets also give off a different energy signature, a phase shift, that tells the researchers the photons are interacting.
"Initially, it was unclear," says Venkatramani. The team had seen two photons interact before, but they didn't know if triplets were possible. After all, he explains, a hydrogen molecule is a stable arrangement of two hydrogen atoms but three hydrogen atoms can't remain together for longer than a millionth of a second. "We were not sure three photons would be a stable molecule or something we could even see," he says.
Surprisingly, the researchers discovered that the three-photon grouping is even more stable than two. "The more you add, the more strongly they are bound," says Venkatramani.
But how do the photons get together? The physicists' theoretical model suggests that as a single photon moves through the cloud of rubidium, it hops from one atom to another, "like a bee flitting between flowers," the press release explains. One photon can briefly bind to an atom, forming a hybrid photon-atom or polariton. If two of these polaritons meet in the cloud, they interact. When they reach the edge of the cloud, the atoms stay behind and the photons sail forward, still bound together. Add more photons and same phenomenon gives rise to triplets.
"Now that we understand what leads to interactions being attractive, you can ask: Can you make them repel each other instead?" says Cantu. Fundamentally, playing with the interaction could reveal new insights into how energy works or where it comes from, he says.
For the purpose of technological advances, photons bound together in this way can carry information—a quality that is useful for quantum computing. And quantum computing could lead to uncrackable codes, ultra-precise clocks, incredibly powerful computers and more. The thing that is so attractive about encoding information in photons is that photons can carry their information across distances very quickly. Already photons speed our communications along fiber optic lines. Bound or entangled photons could transmit complex quantum information almost instantaneously.
The team envisions controlling the attractive and repulsive interactions of photons so precisely that they could arrange photons in predictable structures that hold together like crystals. Some photons would repel each other, pushing apart until they find their own space, while others hold the larger formation and keep the repelling ones from scattering. Their patterned arrangement would be a light crystal. In a light crystal, "if you know where one photon is, then you know where the others are behind it, at equal intervals," says Venkatramani. "This could be very useful if you want to have quantum communication at regular intervals."
The future that such crystals could enable may seem more nebulous than one where people fight with lightsabers, but it could hold advances even more impressive and undreamt of as yet.
Editor's note: This story has been corrected to reflect that photons, not atoms, enter the rubidium cloud and their speed slows while they pass through.