AMPLIFYING AN ATOM WAVE while maintaining its original phase has been demonstrated for the first time, bringing about an atom laser that is the closest equivalent yet to an optical laser. The first atom lasers (Update 305) were passive devices: researchers simply prepared a Bose-Einstein condensate of atoms (Update 233), and then extracted some of the BEC atoms to form a beam.

In the latest round of demonstrations, two research groups (one at MIT and one at the University of Tokyo) have independently demonstrated an atom laser that amplifies its initial beam, in a way that's remarkably similar to how optical lasers augment an initial light wave. Unlike light, however, atoms cannot be created from the vacuum, so researchers must rely on a pre-existing supply of atoms to serve as the initial beam to be amplified.

In the MIT demonstration, researchers shine a pair of laser pulses on a sodium BEC. First, some of the BEC atoms absorb a photon from a high-frequency beam and emit a photon towards a lower-frequency beam. These atoms recoil in the same direction, forming a weak atom wave. Then the lower-frequency beam is shut off, and some of the other BEC atoms absorb light from an intensified pulse coming from the high-frequency laser. The presence of the initial atom wave stimulates these atoms to emit a photon in the direction of the lower-frequency beam. This resulted in a phase-coherent amplified beam about 4 times as strong as the initial atom wave.

The Tokyo group demonstrated similar results with a rubidium-87 BEC. In both demonstrations, the amplification is limited by the size of the BEC, which is depleted in the process. However, an atom-wave amplifier promises improvements in such applications as atom-wave gyroscopes and lithography. (Inouye et al., Nature, 9 December 1999; Kozuma et al., Science, 17 December.)