One-Atom Laser: Trapped atom shoots steady light beam

Talk about miniaturization! California researchers have coaxed laser light from a single cesium atom.

“We’ve pushed [the laser] to its conceptual limit,” says Jason McKeever of the California Institute of Technology in Pasadena. He and his colleagues describe the new device in the Sept. 18 Nature.

For brief intervals, the itsy emitter produces the steadiest stream of laser light ever, Howard Carmichael of the University of Auckland, New Zealand and Luis A. Orozco of the University of Maryland at College Park remark in a commentary that accompanies the Nature article.

Laser emissions with particularly stable intensities and well-spaced photons may prove essential for future computing and communications technologies that exploit the bizarre rules of quantum mechanics (SN: 12/8/01, p. 364: Gadgets from the Quantum Spookhouse), Orozco told Science News. That’s where single-atom lasers might come in. Even a beam lasting only one-tenth of a second might be sufficient for some quantum applications.

Although primitive today, quantum technologies promise to dramatically outperform conventional methodologies in certain ways. Quantum computers, for instance, might eventually search huge databases thousands of times as fast as current machines do.

McKeever says that he and his colleagues are already modifying their single-atom laser to work as a “photon pistol” that shoots a single photon each time it is triggered–a long-sought capability for quantum technologies. Ordinary lasers are more like billion-barrel machine guns emitting vast numbers of photons.

In a laser, material between two mirrors spontaneously emits photons, some of which bounce back and stimulate the coordinated emission of vast numbers of photons. Some of these leak through one mirror to constitute the laser’s beam.

To create the new laser, the Caltech team, led by H. Jeffrey Kimble, mounted a pair of extraordinarily reflective mirrors half a hair’s breadth apart in a vacuum chamber. Then the researchers trapped a single cesium atom in the cavity between the mirrors and chilled the atom to a fraction of a degree above absolute zero. They used a laser-based trapping and cooling technique (SN: 10/25/97, p. 263).

When excited by laser beams entering from outside the cavity, the atom initially emits photons randomly. Almost instantly, however, the atom begins responding to some of its own photons bouncing back from the mirrors. From then on, the lone atom shoots out photons in the direction that the rebounding photons are moving and in synchronization with them. A weak beam of infrared laser light escapes through the mirrors.

For some years, scientists have been making microlasers that use streams of excited atoms. As each atom zips through the space between paired mirrors, photons already bouncing back from the mirrors stimulate it to emit a photon (SN: 12/24&31/94, p. 420).

However, that stimulation is haphazard, says Orozco. In contrast, in the new experiment, the atom is at rest, so “it’s always there with the right disposition,” he adds.

What makes the new laser truly a single-atom device, McKeever says, is that the atom remains confined long enough–roughly a tenth of a second–to emit a beam of photons by itself.

Well, almost. To make the atomic light emitter perform, the Caltech team uses mirrors, optical elements, electronic components, and ordinary full-scale lasers crowded onto a room-size table.


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