To make a degenerate Fermi gas of atoms, physicist Deborah Jin, of the National Institute of Standards and Technology (NIST) in Boulder, Colorado, and her student Brian DeMarco prepared a sample of a Fermi gas of rubidium atoms. To get around the problem of the lack of collisions among very cold identical fermions, they put some of the atoms into a different quantum spin state by applying a radio-frequency electromagnetic field. Since the atoms in the different states were not identical, they could collide and trade energy. Allowing the highest-energy atoms to escape then could cool the whole sample as the remaining atoms collided and redistributed their energy. The sample eventually reached a temperature within a few millionths of a Kelvin of absolute zero, and its approach to the degenerate state is shown in the image with the three peaks.

The emergence of the degenerate Fermi gas of atoms as the temperature is lowered (the image in the foreground has the lowest temperature). The sharpness of each peak indicates the relative number of atoms in the degenerate state. (image courtesy of NIST/JILA)

The group investigating the Fermionic condensate: group leader Deborah Jin, postdoc Markus Greiner, and grad student Cindy Regal (image courtesy of NIST/JILA, copyright Geoffrey Wheeler)

To determine if their cold sample was a degenerate Fermi gas, they turned off the trap and measured how the sample of gas spread out. If the sample were a degenerate Fermi gas of atoms, the sample would spread out rapidly, because atoms had populated relatively high-energy states, since the lower-energy states were already populated and thus unavailable. And that's what the images showed—a rapidly expanding cloud displaying the relatively high energy expected for a degenerate Fermi gas of atoms. More recent experiments have conclusively demonstrated such fermionic condensates.

With this experiment, and the corresponding discovery of the abovementioned BEC, experimenters within five years of each other, from 1995 to 2001, had found atomic systems predicted way back in the mid 1920s, in the great rush of theoretical work that grew out of the newly-developed quantum mechanics. It took experiment so long to catch up with theory because only around the Millennium did low-temperature technology become good enough to make possible experiments so close to absolute zero [see Laser Cooling and Trapping].

Beyond creating a degenerate Fermi gas, physicists want to investigate how the gas atoms interact, and many research groups are at work on this problem. A model of paired electrons explains the remarkable phenomenon of superconductivity [see Super Conductors], and paired helium atoms explain superfluidity, the flow without viscosity of helium at very low temperatures. To test this model, several groups are now actively searching for the superfluid state in an ultracold gas.