Thursday, September 26, 2013

MIT and Harvard create new state of matter: Photonic molecules

MIT and Harvard create new, lightsaber like state of matter: Photonic molecules

The awesomely named Center for Ultracold Atoms, a joint Harvard and MIT venture, has created a new state of matter: photonic molecules. This new state of matter is surprising and interesting, as photons are considered to be massless and incapable of interacting with each other. According to the research group’s leader, who has the unbelievably coincidental surname Lukin, these photonic molecules behave somewhat like lightsabers from the Star Wars universe, with the photons pushing and deflecting each other, but staying linked.
Almost the entirety of our understanding of light is predicated on the knowledge that photons, the elementary particle that makes up the quantum of light and all other electromagnetic radiation, are massless and have no electric charge. If you shine two lasers at each other, because the streams of photons have no mass or charge, the streams of photons simply pass through each other without reacting. It is for this reason that light (and EMR in general) is such a great medium for transmitting data over long distances, and for perceiving visual stimuli with your eyes. If you used almost any other kind of particle to transmit data, it would react violently and fizzle in the atmosphere almost instantly.
Now, however, the Harvard and MIT researchers, led by Lukin, have managed to make photons behave almost as if they’re normal, massive particles. To do this, the researchers pump rubidium atoms into a vacuum chamber, and then cool the vacuum down until it’s a few degrees from absolute zero. Extremely weak laser light — a stream of single photons — is then shone through the rubidium-filled vacuum. As individual photons travel through the medium, it loses energy to the rubidium atoms, slowing down. When the researchers used the laser to fire two photons, instead of one, they found that the photons became a two-photon molecule by the time it left the medium.
A highly informative diagram, showing the attraction (F) between the two photons
A highly informative diagram, showing the attraction (F) between the two photons in a photonic molecule
These photonic molecules have been theorized to exist, through an effect called the Rydberg blockade, but this is the first time that this new state of matter has been physically realized. Unlike a normal molecule, where the constituent atoms are held together by chemical bonds caused by opposite electron or nuclei charges, these photonic molecules aren’t really held together. Basically, as each photon travels through the medium and pushes against the rubidium atoms, they pushed back towards each other, forcing the two photons to coexist. “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.”
As with all new effects, and more so with new states of matter, Lukin and co aren’t entirely sure what practical applications these photonic molecules might have. As we mentioned previously, the way these photonic molecules jostle against each other isn’t completely unlike the way two lightsaber clash in Star Wars. There’s also the fact that photons are our best bet for quantum networking — but performing logic with photons, because they don’t like to interact with each other, is hard. These photonic molecules might provide a solution to this problem. Being an entirely new state of matter, though, we really won’t know what’s possible until we perform a lot more research — which is exactly what the Center for Ultracold Atoms plans to do.