The Mechanism
In 1969 the Soviet physicists Andreev and Lifshitz, and independently the American Gregory Chester in 1970, predicted a state of matter that would be both a crystal and a superfluid: atoms locked in a periodic lattice that nevertheless flowed with zero viscosity. They called it a **supersolid**. For half a century physicists looked for it in solid helium-4 at temperatures fractions of a degree above absolute zero. In 2004 the experimentalists Eun-Seong Kim and Moses Chan at Penn State reported a torsional-oscillator anomaly they interpreted as a possible supersolid signature, in *Nature* — a result Chan and Duk Kim themselves walked back in 2012 after a redesigned experiment showed the original anomaly was due to plasticity in the helium, not to supersolidity. The breakthrough came not in helium but in clouds of magnetic atoms cooled to the nanokelvin regime. In 2019 three independent groups — Pisa, Stuttgart, and Innsbruck — reported the first stable observations of dipolar Bose-Einstein-condensate supersolids: clouds of dysprosium or erbium atoms that, below a critical density, spontaneously broke up into a periodic array of droplets while still phase-coherent across the whole array. The final test was still missing: did the supersolid actually flow without friction? In November 2024 Francesca Ferlaino's group at IQOQI Innsbruck reported the answer in *Nature*. Rotating their erbium cloud with a tilting magnetic field, Eva Casotti and colleagues nucleated quantised vortices — the canonical signature of superfluidity — and saw them embedded in the modulated density landscape of the supersolid lattice. The same atoms were simultaneously a crystal and a frictionless fluid. The press image went out under the title *Tornadoes in a Supersolid*. The 1969 prediction was finally cornered.
Why It Matters
A solid is usually defined by fixed structure, while a superfluid is known for flowing without viscosity. A supersolid seems to combine those opposites: the atoms sit in a regular, crystal-like pattern, yet the whole system keeps one shared quantum phase and can support frictionless flow. The 2024 Innsbruck experiment was important because it did not just see a strange pattern in the density of the atoms. It also produced quantised vortices, which are the classic fingerprints of superfluid motion. That means the same cloud showed both order and flow, something first predicted in 1969 but not cleanly demonstrated for decades.
Wait — That's Not Quite Right
It is easy to think a supersolid must be a normal solid that has melted a little, or that any lumpy cloud of atoms counts as one. That is not enough. The key point is not just a repeating pattern of density, but also superfluid coherence across the whole sample. Earlier helium claims were reinterpreted because the signal came from plastic deformation, not true supersolidity. In the Innsbruck work, the appearance of vortices showed genuine superfluid behaviour inside the patterned cloud.
Vocabulary
- supersolid
- superfluid
- crystal lattice
- zero viscosity
- bose-einstein condensate
- quantised vortices
- erbium
- dysprosium
- nanokelvin
- phase coherence
- dipolar interactions
- absolute zero
Want next week's entry in your inbox?
One short email a week with the latest field guide entry — the fact, the explanation, the quiz, and the activity. Free for parents and teachers.
For adults only · Unsubscribe anytime