A supersolid is a paradox of physics — a material that is both solid and liquid at the same time. This contradictory form of matter was first proposed more than 60 years ago, and, for a long time, people thought it was too nuts to actually exist. But we’re talking about the realm of quantum mechanics, and normal expectations should be thrown out the window.

In 2007, researchers at ETH Zurich and MIT unveiled the world’s first supersolids, starting with superflooding sodium and rubidium, respectively.

Now, an international team of researchers has unveiled an entirely new route to supersolidity, harnessing light-matter particles known as polaritons to create an exotic, flowing crystal.

In other words, this is a supersolid made not from atoms, but from light itself.

“We actually made light into a solid. That’s pretty awesome,” Dimitris Trypogeorgos, a physicist at the National Research Council (CNR) in Italy and a member of the international team behind the discovery, told New Scientist.

What Is a Supersolid, Anyway?

Supersolids are a state of matter that exists only in the quantum realm. They combine the rigid structure of a solid with the frictionless flow of a superfluid. Imagine a crystal that can move without resistance, its atoms hopping freely within a fixed lattice. Physicists have been fascinated by them since the 1950s, when they first theorized that supersolids might exist.

For decades, scientists searched for supersolids in ultracold atoms, particularly in helium-4. In 2004, researchers thought they had glimpsed supersolid helium-4 under extreme pressure, but the results turned out to be a red herring caused by other quantum effects. So, creating a supersolid remained an elusive goal until very recently.

In their new study, researchers across the world took a radically different approach. Instead of using ultracold atoms, they used laser light and a specially designed semiconductor. They fired a laser at a piece of gallium arsenide, a material etched with precise tiny ridges. When the light hit the ridges, it interacted with the semiconductor to create polaritons — quasiparticles (a collective excitation of a large number of particles that behaves as if it were a single particle) that are part light and part matter.

These polaritons were confined by the ridges, forcing them into a crystal-like arrangement. But unlike ordinary solids, this structure also allowed the polaritons to flow without resistance, exhibiting zero viscosity.

The result was a supersolid made entirely of light — a first in the history of physics. “This is really at the beginning of something new,” says Trypogeorgos.

To confirm that their system had entered a supersolid phase, the researchers measured the density of the polaritons. They exhibited a “distinct modulation” in space, as if it were crystallizing. But they also observed signs of coherence — a sign that the system maintained its superfluid character. This means that, despite forming a rigid lattice, the polaritons still moved together in an unbroken wave.

What’s Cool About It?

Light-based supersolids open up new avenues for exploring quantum states of matter. Unlike supersolids made from atoms, which require temperatures close to absolute zero, light-based supersolids might be easier to manipulate and study at room temperature. This could help physicists better understand the nature of supersolids and other exotic quantum phenomena.

Such findings may help physicists better understand how quantum matter can change states. The research also has potential practical implications for quantum technologies, such as ultra-efficient energy transport and novel computing systems.

For now, the supersolid polaritons exist only in a controlled laboratory setting. But with further refinement, researchers hope that this new phase of matter might one day find applications beyond the lab, harnessing the weirdness of quantum physics for future technologies.

The findings appeared in the journal Nature.