Bosons Help Cool Fermi Gases

Roberto Onofrio

Edward A. Hinds

In a gas of identical atoms, each atom is described by a localized de Broglie wave packet. If the gas is trapped and cooled, the atoms move closer together and their de Broglie wavelengths get longer. As the gas is cooled further, the wave packets become larger and eventually start to overlap and interfere with each other. What happens next in these so-called quantum-degenerate gases depends on whether the atoms are bosons or fermions. Quantum gases of both types have proved to be a rich source of new physics in recent years and this trend is continuing with experiments on gases that contain both types of atom, such as the observation of the collapse of a degenerate Fermi gas by researchers at the European Laboratory for Nonlinear Spectroscopy in Florence, Italy (G Modugno et al. 2002 Science 297 2240).

H. A. Buchdahl

Michael R. Andrews

Evaporative cooling of an atom gas of bosons, whereby the hottest atoms are progressively removed, is an effective and widely used method for forming Bose-Einstein condensates. For Fermi gases, however, collisions between identical fermions are forbidden, and new tricks, such as mixing two Fermi gases with different spin states, have been used to form Fermi condensates. Truscott et al. (p. 2570; see the Perspective by O'Hara and Thomas) now create quantum-degenerate Bose and Fermi condensates simultaneously from a boson-fermion mixture. Heat is removed from the fermions through elastic collisions with the bosons, a process called sympathetic cooling. They show that different statistics associated with the two types of gases can actually be visualized.

Wolfgang Ketterle

Experimental methods of laser and evaporative cooling, used in the production of atomic Bose-Einstein condensates have recently been extended to realize quantum degeneracy in trapped Fermi gases. Fermi gases are a new rich system to explore the implications of Pauli exclusion on scattering properties of the system, and ultimately fermionic superfluidity. We have produced a new macroscopic quantum system, in which a degenerate Li-6 Fermi gas coexists with a large and stable Na-23 BEC. This was accomplished using inter-species sympathetic cooling of fermionic 6Li in a thermal bath of bosonic Na-23. We have achieved high numbers of both fermions (less than 10(exp 5) and bosons (less than 10(exp 6), and Li-6 quantum degeneracy corresponding to one half of the Fermi temperature. This is the first time that a Fermi sea was produced with a condensate as a "refrigerator".