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 acts both like a solid and a frictionless “superfluid” at the same time.
The discovery was made by two independent research groups, both reporting in the same issue of Nature magazine. One study was run by a Swiss team at the Institute for Quantum Electronics (IQE) in Zurich, while the other, performed at the Massachusetts Institute of Technology (MIT), was led by Nobel laureate physicist Wolfgang Ketterle.
The studies introduce a new state of matter: one 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 itself is solid, rigidly holding its shape. The coffee is liquid, able to flow. Gas is the air you blow over it to cool it down, while plasma fills the fluorescent tubes in the lighting over your head.
Besides solid, liquid, gas and plasma, there are other states of matter, 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, who won the Nobel prize in 2001 for his work on quantum states of matter.
The bizarre behaviour of liquid helium-4, when cooled to within about two degrees of absolute zero, became apparent in the early 20 th century. Scientists noticed that no open container could hold it — the liquid would flow up the walls. In 1937 Soviet and American physicists independently realised it was a new form of matter: a superfluid.
By the 1950s, physicists were wondering whether an analogous state might exist for solids: a supersolid. Perhaps, if cold enough, helium-4 might solidify into a regular crystal structure.
But after 60 years of trying, nobody was able to observe a supersolid conclusively – until now.
In 2004, for example, scientists thought they’d caught the first glimpse of supersolid helium-4 by freezing it at extreme pressures, but, alas, it turned out to be other quantum effects that produced the weird properties.
The elusive supersolid needs to have two key characteristics. Like a solid, it must retain a rigid structure of atoms. Like a superfluid, atoms within that lattice must be able to hop around between positions without resistance. Both characteristics are reported by the teams in Switzerland and the US.
At MIT, Ketterle and colleagues cooled sodium atoms in a vacuum close to absolute zero, where they became superfluidic. The team then gave half of the atoms a kick with a laser, which put them into a slightly different quantum state than the other half. In this mixture, the two kinds of sodium atoms lined up in a regular way — like in a solid. The team confirmed this by measuring how a laser reflected from it at a particular angle, something that would not occur if the atoms were all jumbled.
Meanwhile the Swiss group, led by Tilman Esslinger, also started with a superfluid, this time of rubidium atoms, which were then arranged into a regular formation using a wave of light bouncing between mirrors. Proof of the supersolid behaviour was subtle, and came by observing a regularity in how the atoms moved.
In both cases, maintaining the supersolid took meticulous care and lots of equipment – a bit different from the scenario first imagined for helium-4, where physicists thought simply getting it cold enough might allow the atoms to form a rigid structure. Still, physicists hope that studying this contradictory new phase of matter might provide insights into superfluids and superconductors.
As Hazzard concludes in a perspective piece for Nature: Every zero-viscosity state of matter so far discovered has been pivotal in expanding physicists’ theoretical concepts and experimental techniques. “The supersolid is sure to be another on this list,” he writes.
