Photons and atoms are both described by a wavefunction, whose phase defines a natural ruler that can be used for measuring fields and forces. But when made ultracold, atoms can offer far greater precision. They also have gravitational signals imprinted on their interference pattern, which can resolve changes in Earth’s gravity to one part in 1011. That’s sensitive enough to detect such features as oil wells, caves, and tunnels; changes in the local water table; and glacial melting. (See the article by Markus Arndt, Physics Today, May 2014, page 30.)
Conventional atom interferometers measure gravity by throwing atoms upward and watching them fall. Light pulses tuned to particular resonance frequencies in the atoms serve as beamsplitters and mirrors. The pulses deliver momentum kicks that split the atoms into two wavepackets, send them along separate paths, and then recombine them. At a detection port, the matter waves...
