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Physicists Use Lasers to Achieve Fermi Degeneracy

Photonics Spectra
Dec 1999
Dr. James P. Smith

It was almost five years ago that Eric A. Cornell and Carl E. Wieman achieved Bose-Einstein condensation at the Joint Institute for Laboratory Astrophysics, cooling bosons to almost absolute zero. Now two other researchers at the same lab, which is operated by the National Institute of Standards and Technology and by the University of Colorado's physics department, have cooled fermions in a similar manner.

As they report in the Sept. 10 issue of Science, Deborah S. Jin and Brian DeMarco have chilled a fermion gas, potassium 40 (40K), to less than 300 nK in laser cooling and magnetic trap experiments. Fermions proved more difficult to bring to this frigid quantum state than their boson cousins.

To achieve the Bose-Einstein condensation, Cornell and Wieman used laser light to hold the atoms, which were trapped in a magnetic field and were further chilled by evaporative cooling. As high-energy atoms leave the trap, the energy of the remaining atoms is redistributed by collisions, leading to cooling.

False-color images show the expanded ultracold Fermi gas at two temperatures, with white indicating the highest density of atoms and blue/black the lowest density. The hotter cloud (left) contains 2.5 million 40K atoms at 2.4 µK, corresponding to T/TF = 3.0. The colder cloud (right) contains 0.78 million atoms at 290 nK, corresponding to T/TF = 0.5. At absolute zero, all of the atoms would lie inside the marked circle, which denotes the Fermi energy.

Achieving Fermi degeneracy -- as it is known -- is more complex because fermions in the same state cannot collide. DeMarco and Jin overcame this obstacle by trapping two different spin states with allowed collisions.

Photonic refrigerator

In the latest experiments, the researchers used 767-nm diode lasers to achieve the first stage of cooling (optical trapping), bringing the temperature of the atomic gas to about 100 µK, Jin said. "We are the first group to do optical trapping and cooling of potassium 40 with an all-diode laser system," she said. The second phase of cooling is done in a magnetic trap, where evaporative cooling occurs. The atomic-gas cloud is then released from the magnetic trap for imaging by a charge-coupled device camera. Other research groups are studying Fermi degeneracy, working with various cooling apparatuses and Fermi gases. Like Bose-Einstein condensation, the fermion equivalent is likely to be a hot topic.

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