Atom Interferometer Measures G with Same Accuracy as Optical Devices

The acceleration due to gravity, g, can be measured simply by timing how long it takes an object to fall. One can accomplish this with great precision by orienting an optical interferometer so that one of its arms is vertical. If the mirror in that arm is then allowed to fall pan way, a Doppler shift in the reflected light signals its rate of fall. A similar measurement can now be done just as accurately with falling atoms (whose atomic frequencies are Doppler shifted), thanks to a long‐term effort by a group at Stanford University. Achim Peters, Keng Yeow Chung, and Steven Chu recently reported that their atom interferometer has determined g to within three parts per billion.