Physicists Have Made the Second-Ever Observation of Interacting Photons


Researchers in a lab at the Massachusetts Institute of Technology were able to link photons and create an entirley new light form that could be used to build light crystals with tremendous scientific applications. 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. Typically, photons don't interact with each other, which is why you don't see light beam bounce off each other - that would be a weird sight. They never appeared to interact with each other, explaining why beams of light intersect and do not reflect with each other. But vaporizing rubidium with a laser and keeping it ultracold creates a cloud the researchers contain in a small tube and magnetize. Through this 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 big takeaway from their research is that photons, which normally don't interact with one another, can be forced to bunch into pairs or triplets when they're passed through a hyper-cooled cloud of rubidium atoms, where the photons bounce from atom to atom like pinballs.

Not only did they discover that the photons could interact, they also found that the typically weightless photons gained mass, albeit a fraction of an electron's mass. But after passing through the cloud, the photons creep along 100,000 times slower than normal.

But the cherry on top of the MIT experimental cake came when researchers documented that the photons emerging from the rubidium cloud came out not as single particles, but as pairs, or, in some cases, triplets.

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The scientists knew the photons must be interacting, since the pairs and triplets of photons give off different energy signatures or phase shifts, the authors reported in the journal Science.

"Initially, it was unclear", says Venkatramani.

However, Sergio Cantu, a Ph.D. candidate at MIT, and colleagues managed the impossible - and they did it with a bit more than flashlights. 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.

"It's completely novel in the sense that we don't even know sometimes qualitatively what to expect", Vuletic says. The observed bunching and strongly nonlinear optical phase are quantitatively described by an effective field theory (EFT) of Rydberg-induced photon-photon interactions, which demonstrates the presence of a substantial effective three-body force between the photons. "The more you add, the more strongly they are bound", Venkatramani noted.

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But how do the photons get together? When a single photon travels through the rubidium cloud, it hops from one atom to another, like a bee foraging from flower to flower. One photon can briefly bind to an atom, forming a hybrid photon-atom or polariton.

If two polaritons are moving through a cloud, they can interact and exit the cloud still bound together. The researchers, therefore, believed that if photons can be made to interact in other ways, they may be harnessed to perform incredibly complex quantum computations extremely fast.

In their new work, the researchers wondered whether interactions could take place between not only two photons, but more.

The findings could put to use in a novel quantum communications system which entangles bound-photons, allowing nearly instantaneous transmission of complex quantum information. Already photons speed our communications along fiber optic lines. "With repulsion of photons, can they be such that they form a regular pattern, like a crystal of light?" Some photons would repeal each other, pushing apart until they find their own space, while others hold the larger formation and keep the repealing ones from scattering. This observation suggests a form of interaction and attraction - also known as entanglement - between photons.

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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.