Quantum degeneracy of a Fermi gas
(a) Cooling a Fermi gas of potassium-40 atoms to ultra-low temperatures reveals its quantum nature. The y-axis shows the mean energy per particle, E, divided by 3kBT - the value expected from classical physics. The x-axis shows the temperature of the gas divided by the Fermi temperature, TF. As the gas cools below TF, the mean energy rises well above the classical value because no more than one fermion can occupy the same quantum state and so the fermions have to fill states of higher and higher energy.
(b) Absorption images of a gas of lithium-6 atoms (fermions) and a gas of lithium-7 atoms (bosons). The fermions have a higher energy per particle, which means that they move faster and further than the bosons. The Fermi gas therefore becomes bigger than the Bose gas as both are cooled in a magnetic trap. (Picture credit: Randy Hulet, Rice University)