If asked to describe the characteristic properties of a gas, most physicists would refer immediately to its diluteness: gases such as air or the helium inside balloons are about 1000 times less dense than liquids or solids. But gases are also characterized by the random motion of their constituent atoms and molecules. This, in fact, is a direct consequence of their diluteness, since the thermal kinetic energy of the particles is much larger than their interaction energy. Such random motion can be quantified using classical statistical mechanics, which provides an extremely powerful description of the gases that we encounter under everyday conditions.
At low temperatures, however, this simple picture of a gas as a collection of randomly moving particles no longer holds. Gases usually undergo a phase transition into a liquid or a solid as they are cooled, but such transitions can be prevented at extremely low densities 100,000 times less than that of air. Here, two-particle collisions keep the gas in thermal equilibrium but the three-body collisions required to form clusters or droplets – which would then grow into a liquid or solid phase – are suppressed. When such gases are cooled to nanokelvin temperatures, they remain extremely dilute, but quantum statistics and interactions profoundly change their properties. For example, they can even undergo a phase transition to a remarkable state of matter called a superfluid, which has no resistance to flow.
Ultracold gases provide a valuable tool with which to study condensed-matter phenomena because the isolated atoms can be observed and manipulated with the precision and control of atomic physics, while the forces between them are accurately known. This is crucial for comparing experimental findings with theoretical models. In particular, ultracold gases can shed light on the underlying mechanisms of superfluidity, which depend fundamentally on the quantum identity of the constituent particles of the gases – i.e. whether they are bosons or fermions.
In the June issue of Physics World, Wolfgang Ketterle and Yong-il Shin tell how gases of fermions are providing researchers with insights into complex systems such as high-temperature superconductivity.
To read the full version of this article – and the rest of the June issue of Physics World – please subscribe to our print edition.