MIT Researchers "Amplify" Atoms

A team of ONR-supported MIT researchers led by Wolfgang Ketterle report in the December 9 issue of Nature that they have created a device that increases the number of particles in a beam of atoms. Furthermore, the atoms that come out of the device are in precisely the same quantum mechanical wave formation as the ones that went in. This "matter wave amplifier" is an analog of the laser. The striking difference is that matter, unlike light, must be conserved.

The MIT device may lead to major improvements in precise sensors for gravity and rotation sensing used in navigation and geological exploration, and in the atomic clocks used in GPS for navigation and many commercial purposes.

"The steady stream of amazing work coming out of the ONR program over the last few years has completely reinvigorated atomic physics and provided the Navy and DoD with a powerful new set of tools to develop next-generation clocks and sensors," said ONR Program Officer Peter Reynolds. "The work is nothing short of spectacular."

Ketterle's research team developed the prototype atom laser in 1997. In that work, the coherent matter waves were all created at once, in a Bose-Einstein condensate or "BEC"-- a new form of matter first observed in a gas by ONR researchers at the University of Colorado at Boulder in 1995 and shortly afterward by Ketterle's group at MIT. Atoms from the BEC were released from a magnetic trap in clumps. Since the coherence of the matter waves produced was analogous to laser light it was much like a pulsed optical laser for atoms. But scientists questioned whether atoms can be amplified, the way light can be in passing through an optical laser. Amplifying atoms is trickier than amplifying the electromagnetic waves that make up light or radio waves, because the number of atoms is conserved while electromagnetic waves can be created from other forms of energy. Yet matter amplification is what the researchers have accomplished.

Working with ultracold atoms whose movements are slowed to a crawl, the MIT group sent a weak beam of sodium atoms through their newly created device. A beam 30 times stronger emerged. The device works by transferring atoms from a reservoir to the input wave, like a stream feeding a river to increase its flow. This transfer of atoms was accomplished with laser light. The recoil of the light scattering process stimulated the acceleration of some atoms from the BEC to exactly match the velocity of the input atoms. Not only were the atoms amplified, "what came out was exactly in lockstep with what we put in," Ketterle said. "The output atomic beam is coherent with respect to the input beam," proving that the device is a matter-wave amplifier or "laser" for atoms. (The term "laser" is an abbreviation for Light Amplification by the Stimulated Emission of Radiation.) According to specialists in the field, such an achievement was not expected for years to come. This marks the birth of active atom optics devices, taking one from such passive devices as diffraction gratings and mirrors to active devices that actually amplify.

The present direct observation of atom amplification was a spin-off, in a sense, of a surprising recent observation by the MIT researchers. They illuminated a BEC with laser light from a specific direction, with a specific polarization. The condensate responded by emitting highly directional beams of atoms and light. Only later did they realize that this baffling phenomenon, which they called a new form of super-radiance, was actually a mechanism for matter wave amplification.

Coherent matter wave amplifiers may improve the performance of atom interferometers by making up for losses inside the device or by amplifying the output signal. Atom interferometers are the matter-wave analogues of optical interferometers, highly precise sensors using the interference of two laser beams, and are already used as precise gravity and rotation sensors.

While applications for the atom laser are still years away, it may someday replace conventional atomic beams where ultimate precision is required, such as in atomic clocks or in tests of the fundamental laws of physics. Atom lasers might be used for high-resolution atom deposition on surfaces for the fabrication of novel materials and nanostructures. The amplification of atoms is a major step in creating and controlling laser-like atom beams.

Ketterle said this experiment is just the beginning. "The rapid developments in the past few years since Bose-Einstein condensates were discovered have taken everybody by surprise. We already are doing experiments that nobody would have imagined just a year or two ago. We have reason to expect further major advances in the near future."

This work had additional support from the National Science Foundation, NASA, the U.S. Army Research Office and the Packard Foundation.

Graphics are available for this release at http://www.onr.navy.mil/onr/newsrel/nr991208images.htm.

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The Office of Naval Research pursues an integrated science and technology program from basic research through manufacturing technologies. Research areas include oceanography; advanced materials; sensors; electronics; surveillance; mine countermeasures; weapons; and surface ship,submarine and aircraft technologies. For more information about ONR programs, refer to the ONR home page at http://www.onr.navy.mil on the World Wide Web.