A Rydberg atom mixer determines a phase of modulated carrier radiation and includes: a reference radiofrequency source for reference radiofrequency radiation; a modulated carrier source for modulated carrier radiation; a vapor cell to contain gas atoms and that receives reference radiofrequency radiation and modulated carrier radiation, such that the gas atoms produce modulated light modulated; and a transmission detector that receives the modulated light from the vapor cell and produces a transmission signal from the transmission detector for determination of a phase of the modulated carrier radiation, wherein the Rydberg atom mixer mixes the reference radiofrequency radiation and the modulated carrier radiation by the gas atoms in a Rydberg electronic state to produce the intermediate frequency IF that corresponds directly to the phase of the modulated carrier radiation.