A sliver of crystalline light sounds like something from romantic poetry or a particularly imaginative fantasy novel. Physicists, however, consider it a possible outcome of an achievement that sounds impressively unlikely on its own; making photons repel each other.
Photons, the units from which light is made up, do not attract or repel each other. They may impart momentum to atoms, for example, but do no such thing to others of their own kind, impeding the possibility of making lightsabers, for example. At least that is the norm, but physicists learned long ago that in the world of quantum behavior calling anything impossible is a risky move. In 2013 one exception was found, with photons traveling through an ultracold gas developing an attraction for each other, as if huddling together for warmth.
Members of the team responsible for that discovery wondered whether, if photons could attract, they could also repel. Seven years later their efforts have been rewarded, described in a new paper in Nature Physics. They've even demonstrated both repulsion and attraction in photon triads.
Sticky or repulsive photons are not something you'll encounter in your everyday life. To make them, MIT's Professor Vladan Vuletić and co-authors had to cool rubidium gas to 50 millionths of a degree above absolute zero and use electromagnetic fields to make the gas transparent. In this environment photons passing through enter what is known as a Rydberg state, resembling an atom with electrons excited enough to approach ionization levels. The researchers then caused two photons to each become coupled to a different atomic state of the same atom.
After shining photons onto the rubidium the team was able to show the probability of finding two photons leaving the gas at the same time was reduced sufficiently so they must be repelling each other. Similarly, by tuning the atoms with a different field, the researchers could show more than chance co-location for the photons, indicating attraction.
Conditions that pushed the photons apart also caused repulsion from both to a third photon when it was added.
The work demonstrates the possibility of fine photon control, so that they are attracted to each other from a distance, but repelled as they get closer, keeping a stable relationship that can be built into something the structure of a crystal.
“This opens avenues to studying exotic phases of matter, including self-organization in open quantum systems, as well as photonic quantum materials,” the paper notes. Photonic crystals could have applications in quantum communication, but we're good with the lightsaber idea too.