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

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: $\frac{\Delta c}{c}\le 3\times {10}^{-9\mathrm{}}$.