Condensates in a whirl
Oct 29, 1999
Tornadoes, waterspouts and other atmospheric vortices are instantly recognizable, but a detailed microscopic understanding of these phenomena continues to elude scientists. The same is true of vortices in superfluids such as helium-4. Superfluids can flow without friction at low temperatures because the bosonic helium-4 atoms all accumulate in the same quantum state, forming a Bose-Einstein condensate. Yet the strong interactions between the helium-4 atoms have hindered in-depth studies of the microscopic nature of superfluidity. Now Eric Cornell, Carl Wieman and co-workers at the JILA laboratory in Boulder, Colorado, have created a vortex in a dilute gas of rubidium-87, another Bose-Einstein condensate (Phys. Rev. Lett. 83 2498).
Physicists have been attempting to create a vortex in a dilute Bose condensate since these systems were first created in 1995. In principle, atomic condensates are ideal systems in which to study vortices because they can now be created and probed routinely using lasers and microwaves (see Physics World August 1999). However, a direct demonstration of the expected superfluid behaviour has proved elusive. Since the vortices seem to form on a longer timescale than the lifetime of the condensate, they cannot be generated by simply "stirring" the system.
The JILA team overcame the problem by simultaneously trapping two "hyperfine" spin states of otherwise identical rubidium-87 atoms, following a proposal by their colleagues Murray Holland and James Williams (Nature 401 568). The first stage involved confining the atoms in a magnetic trap and cooling them with lasers and magnetic fields. The researchers then applied a microwave field to the condensate and focused a laser beam at various points around its circumference, splitting the atoms into two hyperfine states. This set the atoms at the perimeter - which were all in the same hyperfine state - in motion around a stationary core of atoms, which were in the other hyperfine state, thus creating the vortex.
The two states then behaved like two interpenetrating superfluids with a different relative phase. The JILA researchers exploited the quantum-interference effects caused when the atoms in the two states overlapped with each other to image the vortex and measure its properties.
In a separate experiment, physicists at the Massachusetts Institute of Technology observed evidence for a critical velocity - a key characteristic of superfluids - in a Bose condensate of sodium atoms (Phys. Rev. Lett. 83 2502).
Both teams hope that further studies of Bose condensates will eventually lead to a deeper understanding of superfluidity and, possibly, superconductivity.