Test of the isotropy of the one-way speed of light using hydrogen-maser frequency standards

A new test of the isotropy of the one-way velocity of light has been performed using NASA’s Deep Space Network (DSN). During five rotations of the Earth, we compared the phases of two hydrogen-maser frequency standards separated by 21 km using an ultrastable fiber optics link. Because of the unique design of the experiment, it is possible to derive independent limits on anisotropies that are linear and quadratic in the velocity of the Earth with respect to a preferred frame. Assuming that the anisotropies have not been partially cancelled by systematic environmental effects on the instrumentation, the best limits that can be inferred from the data are Δc/c<3.5×${10}^{\mathrm{-}7}$ and Δc/c<2×${10}^{\mathrm{-}8}$ for linear and quadratic dependencies, respectively, on the velocity of the Earth with respect to the cosmic microwave background. The theoretical interpretation of the experiment is discussed.