A collection of fermions — sub-atomic particles with half-integer spin — will not all crowd into one energy state. Rather, two by two, they will populate the lowest-available states up to some cut-off energy, beyond which no ground-state fermions may stray. These populated states are bounded by the so-called Fermi surface. All fermionic systems have a Fermi surface, and an ultracold gas of potassium atoms trapped in an optical lattice is no exception, observe Michael Köhl and co-workers (Phys. Rev. Lett. 94, 080403; 2005).

The lattice is composed of three standing waves of laser light held at right angles to each other. By controlling the depth of the lattice and the particle number, the underlying physics of quantum many-body systems — the interactions between atoms and their dynamics — can be investigated directly.

Köhl et al. show that they can ‘switch on’ interactions between the particles in a Fermi gas. This results in a Fermi surface that, as more fermions are added to the trap, evolves from the smaller, rounded shape seen at the back of the image shown here to the larger, sharp-edged, square pillar in the foreground. Moreover, the interaction between two atoms in different spin states leads to the higher-energy bands becoming populated, although the number of atoms involved is smaller than expected. This strongly interacting regime is not well understood, but Köhl and colleagues' results clearly demonstrate that optical lattices provide another approach to solving outstanding problems in condensed-matter physics.