Supersolid

Some things are hard like a rock. Other things flow like water. A special thing can do both! It stays in a shape. But it can also flow without stopping. Scientists call this a supersolid.

It is a strange state of matter. It is a solid that can flow. It can flow without any friction. This means it moves very easily.

Long ago, people were not sure it could exist. They thought it was impossible. But in 2017, they proved it was real. They used very cold gases.

These gases were super cold. This helped the atoms act in a new way. They formed a solid shape. They still flowed like a liquid.

It is a very cool discovery. It shows how much we can learn.

A supersolid is a very strange state of matter. Most things are either solid or liquid. A solid keeps its shape. A liquid flows. But a supersolid does both! It has a rigid shape like a solid. It also has superfluid properties. This means it can flow with zero viscosity. Viscosity is how much a fluid resists moving. A superfluid flows very easily without stopping.

For a long time, scientists were not sure it existed. They studied helium-4 to find it. In 2004, some tests showed strange results. But later tests in 2012 did not find it. In 2017, things changed. Scientists used Bose–Einstein condensates. These are clouds of atoms that are very cold. They used light to make these atoms form a solid shape. The atoms stayed in a pattern but still flowed.

In 2019, researchers saw this in lanthanide atoms. These atoms made a supersolid on their own. They did not need extra light to help. This gave proof that supersolids are real. Scientists also used an element called dysprosium. They made a 2-dimensional supersolid gas in 2021. In 2024, they even saw quantum vortices. These are tiny swirls in the supersolid.

A supersolid is a very strange state of matter. Most things are either solid or liquid. A solid keeps its shape. A liquid flows. But a supersolid does both! It has a rigid shape like a solid. It also has superfluid properties. This means it can flow with zero viscosity. Viscosity is how much a fluid resists moving. A superfluid flows very easily without stopping.

How does a thing work if it is both solid and liquid? Scientists think it happens through empty spots called vacancies. In a perfect crystal, every spot has a particle. But zero-point energy can create empty sites. These vacancies can move from site to site like waves. Because these vacancies act like bosons, they can form a Bose–Einstein condensate. This happens at temperatures less than a few tenths of a Kelvin. When these vacancies flow, the particles move in the opposite direction. This creates a frictionless flow while the crystal shape stays.

For a long time, people were not sure if this was real. Scientists studied helium-4 for many years. In the 1980s, they used ultrasound to find odd things in solids. In 2004, they saw behavior that looked like a supersolid. They used a tool called a torsional oscillator. It showed a strange rotation that did not fit old models. However, tests in 2012 showed these results might just be changes in the helium. This made it unclear if helium was a true supersolid.

New discoveries finally proved supersolids exist using ultracold gases. In 2017, researchers used Bose–Einstein condensates to make them. They used light and resonators to make atoms form a solid pattern. These atoms stayed in a lattice, which is a repeating structure. In 2019, three groups saw this in lanthanide atoms. These atoms made a supersolid on their own without extra light. This provided the first definitive proof of the supersolid state.

Since then, scientists have found even more ways to make them. In 2021, they used dysprosium to make a 2-dimensional supersolid gas. By 2024, they even saw quantum vortices, which are tiny swirls. In 2025, they saw supersolid behavior in a photonic-crystal waveguide. By 2026, they even showed a room-temperature version using halide-perovskite. These studies show that supersolids can appear in many different systems. It is an exciting new area of physics to explore.

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.