A supersolid is an exotic quantum state of matter that defies classical physics by simultaneously exhibiting two incompatible properties: the fixed, rigid structure of a crystalline solid and the frictionless flow, or superfluidity, typically seen only in liquids at extremely low temperatures. The very existence of a supersolid challenges the traditional understanding that a material must be either a solid, defined by its fixed shape, or a fluid, defined by its ability to flow. Discovered in laboratory settings cooled to a fraction above absolute zero, the supersolid offers physicists a profound window into the collective behavior of atoms governed entirely by the strange rules of quantum mechanics. It stands as a testament to the complex phases matter can adopt under extreme conditions.
The Dual Identity of a Supersolid
The defining feature of a supersolid is the coexistence of long-range spatial order and non-classical rotational inertia within the same material. Spatial order means the atoms are locked into a regular, repeating pattern, known as a crystal lattice, giving the material its rigidity and solid-like shape. This structure allows the supersolid to resist external forces and maintain its form.
Simultaneously, a fraction of the matter possesses superfluid properties, meaning it can flow without any internal friction, or viscosity. This frictionless flow is a macroscopic quantum phenomenon, often demonstrated by a material’s non-classical rotational inertia. When placed in a rotating container, a normal solid rotates completely, but the supersolid’s superfluid component seemingly remains at rest, reducing the total measured rotational inertia of the system.
This dual nature is impossible to explain using classical physics, where a fixed lattice prevents atoms from moving freely. The resolution lies in quantum mechanics, specifically the concept of quantum coherence. In a supersolid, the atoms are not entirely localized to their lattice positions; rather, their wave functions overlap across the entire system.
This delocalization allows the atoms to move coherently, acting as a single quantum entity that can flow through the rigid structure without resistance. The material behaves like a “flowing crystal,” where its density is modulated into a solid-like pattern, yet the collective quantum wave function extends throughout, enabling frictionless motion. This unusual state breaks two symmetries at once: the translational symmetry of a fluid, by forming a periodic structure, and the gauge symmetry of a solid, by allowing superfluid flow.
The Decades-Long Theoretical Search
The concept of a supersolid was first proposed theoretically in the late 1960s. Models suggested that a quantum solid, specifically solid Helium-4, might exhibit superfluid behavior at ultra-low temperatures. This theory relied on the idea that mobile atomic vacancies could exist within the crystal lattice due to quantum zero-point energy, allowing a fraction of the mass to flow without dissipation.
Solid Helium-4 remained the primary candidate for decades, spurring many experimental attempts. The search appeared to bear fruit in 2004 when a team led by Moses Chan and Eunseong Kim reported a landmark experiment using a torsional oscillator. The apparatus showed an unexpected drop in rotational inertia below 200 millikelvin, which was initially interpreted as the first evidence of frictionless flow.
However, subsequent, more careful experiments demonstrated that the observed effect was likely caused by the solid’s mechanical properties, such as stiffening or elastic effects, rather than true superfluid flow. These reinterpreted results led to uncertainty and controversy, effectively retracting the solid Helium-4 claim as definitive proof. The field then shifted away from traditional solids, focusing instead on creating the necessary dual properties using synthetic quantum systems.
Creating and Observing a Supersolid
The definitive realization of the supersolid state was achieved starting around 2017, not in a natural solid, but in highly controlled, ultracold atomic gases. Scientists used Bose-Einstein Condensates (BECs), which are gases of bosonic atoms, such as Dysprosium, Erbium, or Rubidium, cooled to temperatures just a few billionths of a degree above absolute zero. In a BEC, all the atoms occupy the same lowest-energy quantum state, forming a single, coherent matter wave that is inherently a superfluid.
The challenge was introducing a crystalline structure while preserving quantum coherence. Researchers used strong magnetic interactions, known as dipolar interactions, between atoms to create spatial ordering. These long-range forces cause the atoms to spontaneously self-organize into an array of quantum droplets, forming a periodic, solid-like pattern called a density modulation.
The resulting material is a density-modulated superfluid, retaining the ability to flow without resistance while arranged in a regular, stripe-like pattern. Scientists confirmed this dual state by observing the specific evidence for both properties. The solid-like order is confirmed by imaging the periodic density pattern, while the superfluid component is confirmed by observing the formation of quantized vortices when the supersolid is carefully stirred with magnetic fields.
These vortices, which are tiny, swirling whirlpools of frictionless flow, are considered the unmistakable signature of superfluidity. The ability to create and manipulate these exotic states in a clean, synthetic environment provides a robust platform for studying the fundamental physics of matter that is both rigid and free-flowing. The experimental success represents the culmination of a decades-long theoretical search.