Scientists turn light into a 'supersolid' for the 1st time ever: What that means, and why it matters

Supersolids are a strange state of matter defined by quantum mechanics where particles condense into an orderly, crystalline solid but also move like a liquid that has no viscosity. (Viscosity refers to a substance's internal friction, governing how smoothly it flows). Usually, solids don't move on their own, but supersolids change direction and density depending on particle interactions while maintaining an organized lattice structure.

Supersolids require extremely low temperatures to form — usually very close to absolute zero (minus 459.67 degrees Fahrenheit, or minus 273.15 degrees Celsius). Most of the particles have to occupy the lowest energy state available, and heat makes particles jump up and down like excitable toddlers in a ball pit.

If a material is cold enough, the temperature no longer obscures how the particles interact with each other. Instead, the tiny effects of quantum mechanics become the defining factors in how the material behaves.

Imagine the toddlers have gone home and the ball pit has settled into a calm state. Now we can study in peace how the individual components of the ball pit interact with each other to define its characteristics.

Viscosity is a measure of how easily a fluid changes its shape. A fluid with a higher viscosity tends to stick to itself more and, therefore, resist movement, like how syrup moves more sluggishly when poured from a container compared with how water streams from a tap. All fluids, except superfluids and supersolids, have some amount of viscosity.

The best-known example of a fluid with no viscosity is helium cooled to temperatures within a few degrees of absolute zero. Particles aren't completely still at absolute zero; — they wiggle around a little due to the uncertainty principle. In the case of the helium-4 isotope, they wiggle around a lot — enough to make it impossible for a sample of helium-4 to become solid at absolute zero, unless there are about 25 atmospheres' worth of pressure applied to really squish the particles together.

Helium-4's wiggling at absolute zero and other quantum phenomena cause some drastic changes in how the fluid acts. It stops having friction (and, therefore, has no viscosity) and can quickly siphon itself out of containers, among other things.

Supersolids have been made from atomic gases before. However, the new research used a novel mechanism that relies on the properties of "polariton" systems.

Polaritons are formed by coupling photons (light) and quasiparticles like excitons through strong electromagnetic interactions. Their properties allow them to condense to the lowest possible energy state in a similar way to some atomic gases. In other words, light is coupled with matter, and together, they can be condensed into a supersolid.

Supersolids are important to study because they show the effects of tiny, quantum interactions between particles without temperature getting in the way. When we map out the behavior and characteristics of supersolids, we're really looking at how atoms and particles are put together. This teaches us about the world we live in at a fundamental level.

With more research and development, supersolids could be used for quantum computing, superconductors, frictionless lubricants, and applications we haven't even begun to think of yet. There are so many possibilities we have yet to discover — and making a supersolid out of light is a big step forward.

Editor's note: This article was updated on March 31 to correct an error. A previous version said the new study was published in the journal Science, when it was in fact published in the journal Nature.