Photons are sticking together like never before. The individual photons that make up a beam of light normally do not interact, but a team of researchers have clumped three together for the first time, creating a new form of light.
Usually, when you cross two beams of light, they simply pass straight through one another. But Aditya Venkatramani at Harvard University, at the Massachusetts Institute of Technology, and their colleagues found a way to make photons stick together. On a small scale, this means that the two light beams would merge into one at the point where they crossed.
To make photons attract one another, they first prepared a dense cloud of extremely cold rubidium atoms. Then, they shone a weak laser beam into the cloud, sending a few photons in to interact with the atoms.
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When the photons came out the other side of the cloud, they emerged in clumps of two or three, travelling 100,000 times slower than聽the usual speed of light. The team previously discovered it was possible to make photon pairs this way in 2013, but they were not sure whether triplets would be possible. Since then, they have figured out how to stabilise the experiment better and ran it for a longer time in order to find the more rare three-photon states.
Bound fast
The researchers hypothesize that the photons interact with one another within the cloud by first interacting with the rubidium atoms. When a photon is absorbed by an atom, it forms a sort of matter-photon hybrid called a polariton. While two photons cannot normally directly interact with one another, two polaritons are free to interact via the forces that govern the atoms.
These atomic forces are what allow the photons to become bound to one another, and once the photons leave the polaritons and the cloud, they continue to be attracted together in a sort of molecule of light. The whole process takes about one millionth of a second.
鈥淲e send the light into the medium, it gets effectively dressed up as if it were atoms, and then when it turns back into photons they remember interactions that happened in the medium,鈥� says Cantu. 鈥淲e鈥檙e imprinting an atomic behavior into the photons.鈥�
Because the photons remain entangled after passing through the gas cloud, manipulating one of them could affect the other two. This makes these triplets potentially useful for quantum communications and cryptography.
And if the process can be scaled up to larger number of photons, there might be other, more sci-fi applications.
鈥淵ou can make one lightsaber, but if you need two that can crash together in a fight you need the photons to repel each other,鈥� says Venkatramani. He says that one of the team鈥檚 next missions is to figure out how to make photons push each other away.
Read more: Quantum computers can talk to each other via a photon translator
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