Light Quasiparticles Have Been Turned Into A Supersolid For The First Time

In condensed matter physics, things can get pretty weird. When temperatures get close to absolute zero, it is possible to put matter in a state called supersolid, a crystal that can flow without losing energy. Under the right conditions, it is also possible to couple light with excited states of matter, creating quasiparticles – interactions that behave just like particles even though they are not. Researchers have now been able to combine the two in something that pushes the weirdness to a whole new level: solid light!

We should not be imagining something like a magical glowing brick. We are still talking about interactions at a quantum level, but for all intents and purposes, researchers have been able to trap light in a structure that possesses the properties of a special type of solid, and as solids go, this is an exotic quantum state of matter.

This is not only the first time this physics happens with photons instead of atoms but it also involves a completely new mechanism.

Dr Dimitris Trypogeorgos

This state of matter was proposed in the 1960s, but only in the last decade did this move from a theoretical possibility to a physical reality. The investigation of supersolidity involved Bose-Einstein condensates (BECs) – atoms cooled to almost absolute zero to enter a special state of matter. This light supersolid is a whole new beast.

“We proposed and measured a new paradigm of supersolidity. The theme of supersolidity has been explored in ultracold atomic BECs in the last 10 years. This is not only the first time this physics happens with photons instead of atoms but it also involves a completely new mechanism,” lead author Dr Dimitris Trypogeorgos told IFLScience.

To create this intriguing state, researchers used the semiconductor aluminum gallium arsenide. The semiconductor had ridges in it, and a laser was shone on it. The interactions between laser light and the material produced particles called polaritons, and the ridges forced the quasiparticles to move in a certain way. The polaritons were crafted into a supersolid.   

“A supersolid is a completely new exotic phase of matter. It has the paradoxical properties of being a solid while at the same time being able to flow freely around obstacles like a liquid. All phases of matter are formed undergoing a phase transition which breaks a symmetry of the system. For example, water is a liquid which means is translationally invariant, i.e. it looks the same in all directions, but ice is a crystalline solid and it has only discrete translational symmetry,” Dr Trypogeorgos told IFLScience.

“The supersolid is special in that it breaks two symmetries when it forms: the first one to become a superfluid and the second one to become a solid. This is what we were able to do by manipulating interacting photons.”

A superfluid has the ability to flow without friction. Superfluids can’t be kept in an open-lid container as they will climb the sides and escape. They can be seen flowing uphill. A supersolid has those abilities while keeping a crystalline structure. The field of supersolidity is very young and quite unexplored. This breakthrough will help greatly in understanding it.

“Sometimes it is magical to see in physics how the change of language and perspective leads to new insights,” Dr Trypogeorgos told IFLScience.

“We are at the point where we have identified the bare minimum ingredients for the supersolid phase of matter and now we can start adding to it again. Working with photons gives us a whole new toolset to use and brings this physics to the challenging realm of many-body photonic, driven-dissipative physics. This is only the beginning and there is more work to be done to fully characterise the properties of the state of matter we created.”

A paper describing this fascinating breakthrough is published in the journal Nature.

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