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New State of Matter Created by Fusing Photons

Sep 26, 2013 10:10 AM EDT
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Photons – massless particles of light long thought to not interact among one another – have been fused together to create a new state of matter that until now had only been theoretical, according to a new study published in the journal Nature.
(Photo : Yana / Fotolia via Science Daily )

Photons -- massless particles of light long thought to not interact among one another -- have been fused together to create a new state of matter that until now had only been theoretical, according to a new study published in the journal Nature.

A joint team of researchers from the MIT-Harvard Center for Ultracold Atoms managed to coax photons into binding together to form molecules, a feat that runs counter to decades of scientific belief.

Two laser beams, for example, will simply pass through one another if shined towards each other. But the "photonic molecules" behave less like traditional lasers and more like a light saber from the Star Wars films.

"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," said Mikhail Lukin, a physics professor at Harvard University and lead author of the study. "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 quite a while, but until now it hadn't been observed.

"It's not an in-apt analogy to compare this to light sabers," Lukin added. "When these photons interact with each other, they're pushing against and deflect each other. The physics of what's happening in these molecules is similar to what we see in the movies."

However, instead of using the Force to get normally massless photons to bind to one another, the researchers had to rely on old school science and some extreme conditions.

They started by pumping rubidium atoms into a vacuum chamber and used lasers to cool the atoms to just a few degrees above absolute zero. Then, using extremely weak laser pulses, they fired single photons into the cold cloud of atoms.

Lukin said as the photon entered the atom cloud its energy excited the atoms along its path, which caused the photon to slow dramatically. As the single photon moves through the cold cloud of atoms, its energy is transferred from atom to atom, and the energy eventually exits the cloud with the photon.

"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."

Next, the team shot two individual photons into the cloud. But the results were surprising. Rather than exiting as two individual photons, they emerged from the cloud as one.

"It's a photonic interaction that's mediated by the atomic interaction," Lukin said. "That makes these two photons behave like a molecule, and when they exit the medium they're much more likely to do so together than as single photons."

The merged photons represent a new state of matter that science had previously only theorized.

"We do this for fun, and because we're pushing the frontiers of science," Lukin said. "But it feeds into the bigger picture of what we're doing because photons remain the best possible means to carry quantum information. The handicap, though, has been that photons don't interact with each other."

The potential uses for the new state of matter range from advanced quantum computing to perhaps creating entire crystals out of light. The researchers, however, made no mention of the potential of building a working light saber.

"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," Lukin said.

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