All-optical trapping

All-optical trapping of a degenerate Fermi gas has been demonstrated. First created in a magnetic trap (see Physics Today, October 1999, page 17 ), a degenerate Fermi gas consists of fermionic atoms—those with an odd total number of protons, neutrons, and electrons—sufficiently dense and cold that only the lowest trap energy levels are occupied. An all-optical trap has previously been used to confine a Bose–Einstein condensate (see Physics Today, July 2001, page 20 and September 2001, page 79 ). Now, using a stable, high-power CO₂ laser, physicists at Duke University have created a kind of “optical bowl” for lithium-6 atoms: Slowly lowering the bowl’s rim permitted the hottest atoms to evaporate. The researchers then adiabatically recompressed the trap to its full depth, which tightly confined the remaining degenerate gas. In this way, an equal mixture of lithium atoms in spin-up and spin-down states was captured, a feat not possible in magnetic traps. According to the Duke researchers, such equal two-state mixtures are potentially ideal for forming neutrally charged, quasibound pairs of atoms in Fermi gases—something the researchers hope to observe soon. Several groups are pursuing such an atomic-gas analog of superconductivity in different ways. (S. R. Granade et al., Phys. Rev. Lett. 88 , 120405, 2002 http://dx.doi.org/10.1103/PhysRevLett.88.120405 .)