Lasers Replace Magnet Trap for Condensates

Richard Gaughan

Bose-Einstein condensates are unique phases of matter in which particles share the same quantum wave function. They represent a coherent state of matter, analogous to the coherent state of photons created by a laser. They also make the study of quantum behavior -- common to microscopic scales -- accessible on a macroscopic scale.

Bose-Einstein condensates, however, are difficult to produce: Atoms are laser-cooled to nanokelvin temperatures and transferred to a magnetic trap. Then, by slowly reducing the trap's strength, escaping atoms further cool the remaining particles, inducing formation of a condensate.

For the last five years, researchers have suffered through the difficulties associated with using magnetic traps because they have had no alternative. However, a team at the Georgia Institute of Technology has demonstrated another option: replacing the magnetic trap with an evaporatively cooled optical trap.

Michael Chapman, an assistant professor of physics at the institute, helped develop the quasielectrostatic trap by crossing two 12-W beams -- one horizontal and one 45° from vertical -- from a CO₂ laser. Where the beams intersect, he loaded 2 million ⁸⁷Rb atoms, dropped the power in both beams to 1 W within 1 s and ramped power to 190 mW in each beam. The relatively energetic atoms escaped, causing the nearly 35,000 atoms remaining in the trap to rethermalize below the Bose condensate temperature, where they shared a single wave function.

Besides eliminating the need to generate strong magnetic fields, Chapman said his optical trap carries two other advantages: Its 1-s operation is faster than the 10- to 60-s process of magnetic traps, and it can simultaneously cool atoms in many spin states or atoms with no spin at all.

This speed and flexibility could extend the range of constituent materials available for condensates. Candidate materials include atoms with no ground state magnetic moment, such as strontium or barium, and even molecules -- although, Chapman said, molecules will require an efficient precooling scheme.

With the initial success of optical cooling, Chapman now hopes to study the nature of the spin mixtures that are formed in the condensate. "Down the road, it would be enticing to try to create a molecular [Bose-Einstein condensate] in this trap," he said.