Atom Interferometers Prove Their Worth in Atomic Measurements

Thanks to the wave nature of matter, it's possible to build the atomic analog of an optical interferometer, although it's extremely difficult, because the de Broglie wavelengths of atoms are typically 10 000 times shorter than those of light. The feat was accomplished four years ago by a number of groups, each with a different scheme for splitting and recombining the matter waves. (See PHYSICS TODAY, July 1991, page 17.) We are now starting to see the fulfillment of the hopes raised by those first atom interferometers as researchers hone their interferometers to yield ever more precise values of fundamental constants or explore fundamental aspects of quantum mechanics such as Berry's phase. For example, Steven Chu and his colleagues at Stanford University have measured the ratio $h/m$ of Planck's constant to the mass of a cesium atom with a precision of one part in $107,$ although the systematic errors are at the level of 1 part in $106.$ An experiment to improve the absolute accuracy of the $h/m$ measurement to a few parts in $109$ is underway. This ratio is one factor in the determination of the finestructure constant.