Scientists create the first supersolid
Not all solids are created equal. CATHAL O’CONNELL reports.
Nothing is certain in the quantum world, not even the distinction between a coffee cup and the liquid inside it.
After 60 years of trying, scientists have created an elusive and contradictory form of matter: one that simultaneously acts like a solid and a frictionless “superfluid”.
The discovery was made by two groups, both reporting in Nature. One study was run by a team at the Institute for Quantum Electronics in Zurich, while the other, at the Massachusetts Institute of Technology, was led by Nobel laureate Wolfgang Ketterle.
The studies unveil a new state of matter that “marries solid and liquid properties in spectacular fashion”, according to Kaden Hazzard, a quantum physicist at Rice University in Texas, who was not involved in the work.
Your morning coffee reveals the four common states of matter. The cup is solid, holding its shape. The coffee is liquid. Gas is the air you blow over it. Plasma fills the fluorescent tubes in the lighting above you.
Besides these states, there are others, quantum ones where things get a little crazy. A superfluid, for example, flows totally without resistance.
“If your coffee was superfluid and you stirred it, it would continue to spin around forever,” says Ketterle.
Superfluids have been known since 1937, when physicists noted the bizarre behaviour of liquid helium-4, cooled to within about two degrees of absolute zero. No open container could hold it – it would flow up the walls.
Did an analogous state exist for solids: a supersolid? Probably, but for six decades, nobody was able to observe one – until now.
A supersolid needs two key characteristics. Like a solid, it must retain a rigid structure. Like a superfluid, atoms within that lattice must hop between positions without resistance.
Both are reported by the Swiss and US teams. Ketterle and colleagues cooled sodium atoms in a vacuum close to absolute zero, where they became superfluidic. They then gave half the atoms a kick with a laser, which put them into a slightly different quantum state.
In this mixture, the two kinds of atoms lined up in a regular way – as in a solid. This was confirmed by measuring how a laser reflected at a particular angle, something that would not occur if the atoms were jumbled.
Meanwhile the Swiss group, led by Tilman Esslinger, also started with a superfluid, this time of rubidium atoms, arranged into a regular formation using a light wave bouncing between mirrors. Proof of the supersolid behaviour came by observing how the atoms moved.
Physicists hope that the contradictory new matter might provide insights into superfluids and superconductors. Whatever happens, it’s super cool.
‘ BESIDES THESE STATES, THERE ARE OTHERS, QUANTUM ONES WHERE THINGS GET A LITTLE CRAZY. ‘