Gravity may be the law of the land, but the force it applies varies slightly depending on the rocks beneath our feet. In the 3 August Physical Review Letters, researchers report that they have devised a sensitive new scheme for mapping these variations that relies on the quantum mechanical nature of atoms. The device could eventually be useful for searching out new oil and gas deposits, which can be revealed by tiny gravity anomalies.
The scheme builds on the bizarre dual nature of matter, which behaves--so says quantum mechanics--as solid particles at some times while resembling light waves at others. Instruments called interferometers split light waves, allow them to travel separately for a distance, and then recombine them. The result is a shadowy interference pattern, created because waves that converge in phase create light patches and those that cancel each other out form dark areas. In 1991, physicists showed that "matter waves" of atoms can produce the same effect. Typical atom interferometers work by dropping a collection of ultracold cesium atoms down a vacuum tube while pushing them apart and then back together with laser pulses. Gravity, by acting on the cesium atoms, affects the interference between the moving "atom waves."
In the new experiment, Yale University physicist Mark Kasevich and his colleagues stacked one atom interferometer atop another in order to measure the difference in gravity's pull between them. Using two interferometers allowed the researchers to pick out and throw away distorting effects, such as vibrations, that affected both devices equally. By comparing the results between the two interferometers, the researchers were able to gauge changes in the pull of gravity as small as one part in 10,000,000, a number they have since honed to 30 parts in a billion.
The new work is "a beautiful experiment," says Daniel Kleppner, an atomic physicist at the Massachusetts Institute of Technology. For now the new interferometer-based gradiometers are still not as accurate as the mechanical gradiometers used to look for oil and gas deposits, among other things. Part of the trouble, says Yale team member Jeff McGuirk, is that some vibrations can cause the laser pulses to travel a different path through one interferometer than the other, adding background noise to the experiment. But McGuirk adds that the group has already tested a scheme for compensating for the vibrations, which should improve the sensitivity by another factor of 100 to 1000, good enough to beat the competition.