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Physics Today

Trap Holds Condensates of Two Different Spin States at Once

Barbara Goss Levi

Published Mar 1, 1997• 1 citation

Abstract

Everyone likes a bargain, and what could be better than two for the price of one? That's what researchers in Boulder, Colorado, must have figured when they snared not just one but two spin states of the rubidium atom in an ultracold trap and simultaneously formed overlapping Bose–Einstein condensates from them. The feat—accomplished by Eric Cornell, Carl Wieman and their colleagues at JILA, the University of Colorado and the National Institute of Standards and Technology in Boulder—gives a valuable tool to those interested in atomic collisions and their dependence on spin states, as well as to those interested in further exploration of interactions between and within condensates. As William Phillips of NIST in Gaithersburg, Maryland, points out, such a double condensate, of two different internal states of the same atom, is unlike anything seen in superfluid He4, a condensed-matter Bose–Einstein condensate.

DOI: 10.1063/1.881726

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Everyone likes a bargain, and what could be better than two for the price of one? That's what researchers in Boulder, Colorado, must have figured when they snared not just one but two spin states of the rubidium atom in an ultracold trap and simultaneously formed overlapping Bose–Einstein condensates from them. The feat—accomplished by Eric Cornell, Carl Wieman and their colleagues at JILA, the University of Colorado and the National Institute of Standards and Technology in Boulder—gives a valuable tool to those interested in atomic collisions and their dependence on spin states, as well as to those interested in further exploration of interactions between and within condensates. As William Phillips of NIST in Gaithersburg, Maryland, points out, such a double condensate, of two different internal states of the same atom, is unlike anything seen in superfluid He4, a condensed-matter Bose–Einstein condensate.

The earliest experiments on gaseous Bose–Einstein condensates established that they are coherent collections of atoms, all described by a single wavefunction—just like the known superfluids, liquid helium-4 and superconductors. It was natural to wonder if they would also exhibit some of the behavior that is so characteristic of those larger-scale Bose condensates, such as quantized circulation (vortices) or frictionless flow. Many theorists have proffered ideas on how one would produce vortices in a Bose–Einsten condensate and what their properties might be. One of those ideas has now been experimentally realized. The approach was suggested by James Williams and Murray Holland of JILA and implemented by their experimental colleagues there. (JILA is a jointly operated lab of the University of Colorado at Boulder and the National Institute of Standards and Technology. Williams is now at the University of Toronto.)

The phenomenon of Bose-Einstein condensation (BEC), first predicted by Einstein 70 years ago, has finally been observed in the laboratory. In this elusive state of matter the concept of individual particles ceases to have meaning: the atoms condense into the same quantum state and act as a single entity that displays quantum mechanical behaviour on a macroscopic scale. The condensate was observed when Eric Cornell, Carl Wieman and colleagues at the Joint Institute for Laboratory Astrophysics in Boulder, Colorado, cooled a dilute gas of rubidium atoms, confined in a magnetic trap, to a temperature of just a few nanoKelvin (M H Anderson et al. 1995 Science 269 198).

Bose–Einstein condensates (BECs) were formed five years ago from rubidium-87 and sodium atoms, whose interactions are repulsive. BECs were also formed from lithium-7 atoms, which attract one another. (See the article by Wolfgang Ketterle in PHYSICS TODAY, December 1999, page 30.) Now researchers in Boulder, Colorado, have created a single condensate of rubidium-85 atoms, which can be taken continuously from the repulsive to the attractive regimes. The experimenters can vary the strength of the interaction between the atoms in their condensate by a factor of 100 or more—and even flip its sign—by changing the external magnetic field. The ability to tune the interactions of a condensate opens the door to systematic explorations of atomic behavior in new regimes.

Tornadoes, waterspouts and other atmospheric vortices are instanriy recognizable, but a detailed microscopic understanding of these phenomena continues to elude scientists. The same is true of vortices in super-fluids such as helium-4. Superfluids can flow without friction at low temperatures because the helium-4 atoms, which are bosons, 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 (M Matthews et at. 1999 Plys. Rev. Lett. 83 2498).

Lately there has been rapid progress in techniques for trapping samples of neutral atoms at elevated densities and extremely low temperatures. Now a group at the Joint Institute for Laboratory Astrophysics in Boulder, Colorado, has used a new type of magnetic trap to confine rubidium atoms at the lowest kinetic temperature yet recorded for an atomic gas. Compelling motivations for this effort include the use of such ultra-cold gases in high-precision spectroscopy, low-energy tests of the "standard model" of particle physics, and studies of atomic collisions at very low relative velocities. Particularly exciting is the prospect of studying quantum collective phenomena such as Bose-Einstein condensation (p37).