Supersolid

A supersolid is a unique state of matter found in condensed matter physics. It combines two properties that usually do not go together. First, it has a spatially ordered structure, which is the defining trait of a solid. Second, it possesses superfluid properties, meaning it can flow with zero viscosity. Viscosity is a measure of how much a fluid resists movement. In a superfluid, there is no resistance to flow at all. This state challenges our basic intuition about how matter behaves. Usually, we think of solids as rigid and fluids as flowing. A supersolid manages to be both at the same time.

Scientists use different theories to explain how this dual nature works. One common theory involves the role of vacancies in a crystal. A vacancy is an empty site that would normally be occupied by a particle. In an ideal crystal, every site is full, but zero-point energy can create these gaps. These vacancies can move from one site to another like waves. Because these vacancies behave as bosons, they can undergo Bose–Einstein condensation. This occurs at temperatures below a few tenths of a Kelvin. When these vacancies flow, the actual particles move in the opposite direction. This creates a frictionless superflow while the crystal structure remains intact.

Another way a supersolid can form is by emerging from a superfluid. In this case, the atoms are already in a superfluid state. A spatially ordered structure then forms as a modulation on top of the superfluid density. This process was seen in experiments using atomic Bose–Einstein condensates. There is also a related hypothetical phase called a superglass. A superglass would have superfluidity combined with a frozen, amorphous structure. Anthony James Leggett proposed this idea in 1970. In 2009, researchers theorized that frozen helium-4 might actually be a superglass.

For many years, it was unclear if a true supersolid could exist in nature. Scientists spent decades studying helium-4 to find evidence. In the 1980s, ultrasound experiments detected an anomaly in a solid. By 2004, researchers observed supersolid-like behavior using a torsional oscillator. They noticed a non-classical rotational moment of inertia, which means the rotation behaved strangely. This suggested that a small percentage of helium atoms were acting like a superfluid. However, later studies suggested this might not be a true supersolid. In 2012, repeated experiments showed the behavior might just be changes in the helium's elastic properties.

Definitive proof for the supersolid state finally arrived through ultracold quantum gases. In 2017, researchers used different methods to create supersolidity in Bose–Einstein condensates. One method used two optical resonators to enhance atomic interactions. This caused the atoms to spontaneously crystallize into a lattice supersolid. In this version, atoms are pinned to a specific lattice structure. Another method used light beams to create spin–orbit coupling in a double-well potential. This created a characteristic density modulation through interference. These experiments showed that supersolidity is a real and measurable state.

Recent years have seen even more specific discoveries in this field. In 2019, three research groups independently observed supersolidity in dipolar Bose–Einstein condensates. These were made from lanthanide atoms. In these systems, supersolidity comes directly from atomic interactions without needing an external lattice. This allowed scientists to directly observe superfluid flow. In 2021, researchers used confocal cavity quantum electrodynamics to create a supersolid with lattice phonons. These are vibrations that travel at the speed of sound. By 2024, teams reported observing quantum vortices, which are tiny swirls, in a supersolid phase.

New platforms are now being used to study these strange states of matter. In 2025, supersolid behavior was found in an exciton–polariton condensate. This occurred in a photonic-crystal waveguide. This showed that supersolidity can emerge in a driven-dissipative, non-equilibrium platform. By 2026, a room-temperature supersolid was demonstrated using a halide-perovskite nanophotonic structure. These exciton–polariton systems are a new way to study supersolidity. They complement the work being done with ultracold atomic gases. This research continues to expand our understanding of how matter can be organized.