At sufficiently low temperatures and sufficiently high densities, certain types of atoms form a Bose–Einstein condensate. In this peculiar phase of matter, the wavefunctions of all the atoms in a system become identical and extend throughout the container in which the atoms are held. The position of an atom inside the container is therefore not defined, and the probability of finding the atom is the same at all points.
Mathematically, it is easy to describe the process of Bose–Einstein condensation for a weakly interacting system of particles, such as an ideal gas. Actually creating such a condensate is more difficult, but this was finally achieved in 1995 for a gas of ultracold rubidium atoms.
Bose–Einstein condensation in the liquid phase has been around for much longer. Superfluidity – the state in which a fluid flows with zero viscosity – was discovered in liquid helium-4 at 2.2 K in the 1930s and in helium-3 at the much lower temperature of 3 mK in 1972. As far as we know, superfluidity is a unique consequence of Bose–Einstein condensation, although condensation does not necessarily produce a superfluid.
The theoretical description of the condensation process in liquids is much more complicated than for an ideal gas because the atoms are strongly interacting. For decades researchers have speculated that Bose–Einstein condensation could also occur in a system of spatially localized atoms – in other words in a solid. Now it seems that such a “supersolid” phase may have been observed in solid helium-4. Eun-Seong Kim and Moses Chan at Pennsylvania State University claim that solid helium-4 can behave as if part of it is a superfluid, even though atoms in the solid are known to be localized in the regular array of a crystalline lattice (Nature 427 225). If confirmed, their results will mean that Bose–Einstein condensation has now been achieved in the gas, liquid and solid phases.
In the March issue of Physics World John Goodkind in the Physics Department at the University of California at San Diego describes this work in more detail.