30th September 2013

New form of matter behaves like a Star Wars lightsaber

Photonic molecules, until now considered purely theoretical, have been created by researchers at Harvard and the Massachusetts Institute of Technology.

Working with colleagues at the Harvard-MIT Center for Ultracold Atoms, a group led by Harvard Professor of Physics Mikhail Lukin and MIT Professor of Physics Vladan Vuletic managed to coax photons into binding together to form molecules. Their work is described in the journal Nature.

The discovery, Lukin said, runs contrary to decades of accepted wisdom about the nature of light. Photons have long been described as massless particles that don’t interact with each other. Shine two laser beams at each other, he said, and they simply pass through one another.

Photonic molecules, however, behave less like traditional lasers and more like something you might find in science fiction: the light saber.

“Most of the properties of light we know about originate from the fact that photons are massless, and that they do not interact with each other,” Lukin said. “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. This type of photonic bound state has been discussed theoretically for a while, but until now it hadn’t been observed.

“It’s not an inapt analogy to compare this to light sabers,” he continued. “When these photons interact with each other, they’re pushing against and deflecting each other. The physics of what’s happening in these molecules is similar to what we see in the movies.”

To get the normally massless photons to bind to each other, the researchers began by pumping rubidium atoms into a vacuum chamber, then used lasers to cool the cloud of atoms to just a few degrees above absolute zero. Using extremely weak laser pulses, they fired single photons into the cloud of atoms. As the photons entered the cloud, they bumped into the rubidium atoms in a way similar to how regular matter would. In the process, the photons slowed down dramatically – enough to bump into each other and bond into molecules.

“When the photon exits the medium, its identity is preserved,” Lukin said. “It’s the same effect we see with refraction of light in a water glass. The light enters the water, it hands off part of its energy to the medium, and inside it exists as light and matter coupled together. But when it exits, it’s still light. The process that takes place is the same. It’s just a bit more extreme. The light is slowed considerably, and a lot more energy is given away than during refraction.”

“What it will be useful for, we don’t know yet. But it’s a new state of matter, so we are hopeful that new applications may emerge as we continue to investigate these photonic molecules’ properties,” he said.

Lukin believes it could improve the performance of quantum computing operations. He also speculated that the system might one day be used to generate complex, 3D structures – such as crystals – wholly out of light.

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