Ultracold atoms: new gravity yardstick?
A prototype of a novel, laser-cooled device for measuring gravity, described in the Aug. 3 Physical Review Letters, promises to be a boon for oil exploration, geophysical measurements, and military uses, says Mark A. Kasevich, the Yale University physicist leading the U.S. Navy-funded development team. "I think there's a possibility for a basic science idea to have an impact technologically," he says.
Nobel prize-winning techniques developed in the 1980s to slow, and thus cool, atoms by zapping them with laser-generated photons have led to stunning advances in physics, including the creation of Bose-Einstein condensates, which are ultracold clusters of atoms sharing one quantum state (SN: 7/25/98, p. 54). The only practical gadget to emerge from the field, however, has been a better atomic clock, attractive only to a few time-standard labs, says Steven L. Rolston at the National Institute of Standards and Technology in Gaithersburg, Md.
The newer instrument determines gravity's gradient, or change in strength with position, by comparing the gravitational acceleration of two clouds, each made up of millions of cesium atoms. The clouds are cooled to 3 microkelvins and spaced a meter apart. Observing interference within each cloud's quantum-mechanical wave behavior yields a precise measurement of gravity at that position.
"It's beautiful work," Rolston says. "It's nice to see something showing some true practicality."
Measurement of gravity-gradient changes over an area can reveal subsurface irregularities in mass, which may represent underwater mountains or subterranean oil deposits. The Navy already uses precision-machined electromechanical gravity gradiometers to help submarines navigate without noisy sonar. In recent years, it has made the once-classified instruments available to civilian geologists.
The quantum-mechanical gradiometer has yet to surpass electromechanical instruments in sensitivity. Its advantage lies in its remarkable stability, derived from using fundamental properties of atoms and fixed laser frequencies as references, Kasevich says. In contrast, electromechanical gradiometers require periodic calibration because their components are vulnerable to temperature change and other influences.
Gravity surveyors would welcome an instrument whose measurements didn't drift, says Richard O. Hansen of Pearson, deRidder and Johnson, a geophysics consulting company in Lakewood, Colo. The quantum-mechanical design is "gorgeous," he says, but he cautions that a field-deployable version must maintain its laboratory performance while remaining small, light, and inexpensive.
Ultimately, the quantum-mechanical gradiometer should achieve a 10-fold to 100-fold edge in sensitivity over electromechanical versions, Kasevich says. On the other hand, he acknowledges that it has so far passed only a "proof of principle" test, with its first field trials anticipated within a year.