Doppler Beats in Superradiance

We report observation of modulations in superradiance, resulting from the beating of light fields emitted by two groups of atoms with different velocities. The beat note is displayed from atomic frequency by the Doppler shift corresponding to the velocity difference (red or blue shift, depending on the direction of superradiant emission). Similarly many-atom beats should occur in superradiance from a mixture of different isotopes. Implications in spectroscopy are considered.