Fermions go degenerate
Sep 10, 1999
When a gas of atoms is cooled to very low temperatures its behaviour depends on whether the atoms are bosons or fermions. Bosons can all occupy the same quantum state, whereas the exclusion principle prevents two identical fermions from having the same quantum numbers. In 1995 physicists at the JILA laboratory in Boulder, Colorado, cooled a gas of rubidium atoms, which are bosons, to temperatures so low that all the atoms condensed into the same quantum state. The creation of this Bose condensate made headlines around the world and sparked off an intense period of experimental and theoretical activity that continues today. Now another team at JILA has reported the first evidence for such quantum degenerate behaviour in a gas of fermionic atoms (Science 285 1703).
Brian DeMarco and Deborah Jin used a pair of magneto-optical traps to confine about 700000 atoms of potassium-40 at temperatures below 300 nanokelvin. This is about half of the degeneracy temperature for a gas of fermions. At these temperatures the occupation of the lowest quantum states increases from around zero to about 60%. The quantum degeneracy was observed as a barrier to evaporative cooling of the sample and a change in its thermodynamic behaviour. Measurements of the momentum distribution and total energy of the gas also revealed its quantum statistics.
A big challenge faced by DeMarco and Jin was cooling the gas. Most cooling techniques rely on collisions between the atoms for cooling, but since two identical fermions cannot be in the same place at the same time, the collisions needed for cooling cannot happen. The NIST team overcame this problem by placing the atoms in different magnetic sublevels. Atoms in the different sublevels were no longer identical and could therefore collide.