Bose-Einstein condensate forms "atom laser"

A research grou¥at the Massachusetts Institute of Technology (MIT; Cambridge, MA) has demonstrated an "atom laser" produced from a Bose-Einstein condensate of sodium atoms. As with the optical analog, the atoms emitted are coherent, forming a single matter wave.

Bose-Einstein condensate forms "atom laser"

Rick DeMeis

A research grou¥at the Massachusetts Institute of Technology (MIT; Cambridge, MA) has demonstrated an "atom laser" produced from a Bose-Einstein condensate of sodium atoms. As with the optical analog, the atoms emitted are coherent, forming a single matter wave.

The Bose-Einstein condensate forms around 1 ¥ 10-6 K and is obtained by cooling a gas to such low temperatures that the atomic-matter waves overlap, with the atoms losing their individual identities. The single entity exhibits uniform behavior, as opposed to atoms in a gas that move independently. The MIT grou¥cooled the gas in two stages. The first used laser beams with frequencies and polarizations such that photons emitted by the gas atoms exposed to the beams were slightly more energetic than those absorbed by the atoms. This energy transfer cooled the atoms to around 100 ¥ 10-6 K. The Zeeman effect was next used to confine the atoms in a magnetic tra¥within an ultrahigh-vacuum chamber. Hotter atoms in the higher-energy "shells" were expelled by flipping their magnetic moments with radio-frequency energy.

To form a laser from such a condensate, a controlled means of extracting a beam of atoms and a method of determining whether the atoms in the beam are coherent was needed. In July 1996, the researchers first extracted a condensed-atom beam from a trapped condensate by applying an oscillating magnetic field. The "laser" emitted multiple pulses of Bose-condensed droplets consisting of hundreds of thousands to millions of atoms (see figure).

What remained to be demonstrated was coherence of the pulses acting as a single wave. Grou¥leader Wolfgang Ketterle notes, "For photons it`s much easier to show coherence. Photons don`t interact with each other, they are not affected by gravity, and they are not affected by molecules in the air."

In November of last year, the researchers were able to view coherence using standard techniques that show interference between two waves--a periodic standing wave that could be photographed. After aligning and focusing two overlapping Bose-condensed sample pulse trains, they saw the interference pattern with a wavelength of 15 µm--extremely large for atomic-matter waves where room-temperature atoms exhibit a matter wavelength of only 0.04 nm. A dual-laser-beam condensate absorption tomography imaging technique was used in which a 100-µm laser beam focused into a sheet "selects" a slice of atoms whose interference fringes effectively generate a diffraction grating. Another laser beam at right angles to the first is passed through the slice of atoms to image the interference fringes.

Future atom-laser applications might include directly depositing atoms onto microcircuit substrates, producing extremely fine circuit elements. The MIT work was funded by the US Office of Naval Research, the National Science Foundation, the Army Research Office, and the Packard Foundation.

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