Image: TKTK

Atoms come in two basic flavorsbosons and fermionsand for the first time physicists have cooled a mixed gas of the two to such low temperatures that their fundamental differences are readily visible. The mixture contained lithium-6 and lithium-7 atoms.

The first kindwhich contain three neutrons, three protons and three electronsare fermions, so designated for their odd number of parts. Lithium-7 atoms, which bear an even number of parts thanks to an extra neutron, are bosons. At ultra-cold temperatures both types become more like waves than particles, a state known as quantum degeneracy. But how quantum degeneracy affects their properties is very different.

Indeed, identical fermions begin to avoid each other, and thus reach a point beyond which they cannot be compressed. This pressure, called Fermi pressure, is what stabilizes cold, dead starsnamely white dwarfs and neutron starsfrom collapsing under their own gravitational attraction. Identical bosons, on the other hand, begin to act like a single, unified superatom, known as a Bose-Einstein condensate. Randall Hulet of Rice University and other scientists first demonstrated these condensates in 1995.

Now Hulet and his colleagues cooled both lithium-6 and lithium-7 atoms to temperatures around 240 nanokelvins, less than one fourth of a millionth of a degree above absolute zero. They created images of the two gases independently, recording their striking differences. Whereas the size of the fermion gas hit a lower limit, the size of the boson gas continued to shrink (see image). The team is working on cooling the gas even further, in hopes of creating correlated fermions called Cooper pairs. In the meantime, Hulet says that "this will enable exploration of the underlying theory of superconductivity, and by adjusting the strength of the interactions between atoms we hope to understand its implications in a way not previously possible."