Test of the Isotropy of the Speed of Light Using Fast-Beam Laser Spectroscopy

Abstract

We report on a novel experiment sensitive to the anisotropy of the one-way speed of light. The frequency of a two-photon transition in a fast atomic beam is compared to the frequency of a stationary absorber while the direction of the fast beam is rotated relative to the fixed stars. The experiment yields an improved upper limit for the anisotropy: $\frac{\ensuremath{\Delta}c}{c}\ensuremath{\le}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$.