Atom interferometry and the gravitational redshift

Restricted Access. An open-access version is available at arXiv.org (one of the alternative locations)From the principle of equivalence, Einstein predicted that clocks slow down in a gravitational field. Since the general theory of relativity is based on the principle of equivalence, it is essential to test this prediction accurately. M¨uller, Peters and Chu claim that a reinterpretation of decade old experiments with atom interferometers leads to a sensitive test of this gravitational redshift effect at the Compton frequency. Wolf et al dispute this claim and adduce arguments against it. In this paper, we distill these arguments to a single fundamental objection: an atom is not a clock ticking at the Compton frequency. We conclude that atom interferometry experiments conducted to date do not yield such sensitive tests of the gravitational redshift. Finally, we suggest a new interferometric experiment to measure the gravitational redshift, which realizes a quantum version of the classical clock ‘paradox’. PACS numbers: 04.80.−y, 03.75.D