Quantum gases of both types have proved to be a rich source of new physics in recent years and this trend is continuing with experiments on gases that contain both types of atom, such as the observation of the collapse of a degenerate Fermi gas by researchers at the European Laboratory for Nonlinear Spectroscopy in Florence, Italy (G Modugno et al. 2002 Science 297 2240).
If the atoms in the ultracold gas are bosons - that is if their spin quantum number is an integer - they will all collapse into same quantum ground state. This new state of matter, which is known as a Bose--Einstein condensate, is described by a single de Broglie wave packet that fills the whole volume of the condensate. As long as the interactions between the atoms are repulsive then the condensate will be stable. However, the condensate will normally collapse if the atoms attract each other. In recent years Bose condensates have proved to be marvellous systems for studying quantum fluids and their fundamental properties, particularly their macroscopic quantum phase, and experimenters have observed Josephson oscillations, superfluidity and interference fringes between matter waves from condensates.
However, if the atoms in the gas are fermions the gas will behave completely differently. Unlike bosons, which are gregarious, fermions prefer to keep their distance because the Pauli exclusion principle prevents two identical fermions from occupying the same quantum state. This leads to a very strong effective repulsion between identical Fermi atoms.
This "Fermi pressure" means that a trapped Fermi gas will have a relatively large size and kinetic energy, even at temperatures close to absolute zero. This process also stabilizes white dwarfs and neutron stars against gravitational collapse. However, Massimo Inguscio and co-workers in Florence have now shown that a Bose condensate embedded within a degenerate Fermi gas can trigger the collapse of the gas.
In the November issue of Physics World, Ed Hinds of Imperial College and Sussex University explains how the experiment would tell us more about superfluidity - and maybe even superconductivity