American researchers have been able to create a three-photon bound state that represents a completely new form of light. You may know that light can interact with itself creating diffraction patterns, but this new interaction has photons behaving like atoms, sticking together to create a new state.
As reported in Science, the researchers have been looking at complex interactions between photons for years. They obtained a two-photon state in 2013, creating a new state of matter. Their success pushed them to investigate whether more complex bound photon states were possible.
“For example, you can combine oxygen molecules to form O2 and O3 (ozone), but not O4, and for some molecules, you can’t form even a three-particle molecule. So it was an open question: Can you add more photons to a molecule to make bigger and bigger things?" senior author Professor Vladan Vuletic, from MIT, said in a statement.
"What was interesting was that these triplets formed at all. It was also not known whether they would be equally, less, or more strongly bound compared with photon pairs.”
To obtain the newly observed bound state, the scientists had to cool a cloud of rubidium atoms to ultracold temperature, just one-millionth of a degree above absolute zero. Temperature is a way to express the energy of atoms and molecules. These atoms have very little energy, so they are pretty much stuck in place in this cloud.
The researchers shot a weak laser beam through the cloud so that only a small number of photons are in the clouds at any given time. When the photons come out of the cloud they are no longer single but they are either in pairs or triplets. (Whatever is happening in that cloud is a lot more efficient than tinder.)
Although quantum online dating is a hypothesis, researchers have a better one. Photons in the ultracold cloud are likely to jump from one atom to another. There they can bind to rubidium and form a polariton, a special interaction between photons and atoms. These polaritons can interact with each other and the state can move around from atom to atom. Eventually, these polaritons' interactions reach the edge of the cloud, where they leave the atoms behind moving forward in a photon bound state. The researchers discovered this can also occur with three photons.
The team hopes to see next if they if they can make the photons exhibit repulsion, where they would bounce off each other.
“It’s completely novel in the sense that we don’t even know sometimes qualitatively what to expect,” Vuletic explained. “With repulsion of photons, can they be such that they form a regular pattern, like a crystal of light? Or will something else happen? It’s very uncharted territory.”
The researchers believe that state could be extremely useful in quantum computing and information technology.