Kastler Brossel Laboratory, CNRS, France.
A quantum-gas specialist learns about crystals from his own science.
Crystals can behave as electrical insulators or conductors. In a few crystals and under the right conditions, electrons flow perfectly. And in a subset of these superconducting crystals, the minimum temperature for perfect conduction is bizarrely warm.
On the whole, physicists have tried to explain this using models with a small number of parameters, such as the probability of an electron jumping between two sites, and the interaction energy between two neighbouring electrons. Extensive laboratory studies measuring every conceivable property of the curious crystals confirm several predictions of these models, but their general solution is still hotly debated.
Recently, a couple of research groups have been casting around for less obvious ways to understand superconducting crystals, and turned to the field that is my bread and butter: quantum gases. They have modelled electrons zooming through these crystals using gases of cold potassium atoms moving around in a space demarcated by laser beams — a kind of egg box made with light.
In December, a group led by Immanuel Bloch detected cold potassium gas switching to a state with exactly one atom per compartment of the egg box. Such an ordered state is considered a key ingredient for superconductivity. Bloch’s team was not the first to see the switch, but the group’s measurement of the size of the gas revealed a crucial property of this phase: its incompressibility (U. Schneider et al. Science 322, 1520–1525; 2008).
This means that quantum gases are insulators as well as conductors, making the experimental analogy to superconducting crystals more complete — and making them more useful playthings for scientists studying superconducting crystals.