Autonomously measuring an atomic clock´s allan variance

In this work, we demonstrate that an atomic clock can autonomously assess its own frequency stability and integrity by comparing the phase of its output signal to a delayed version of itself in what is essentially an interferometric technique. Using a high-quality crystal oscillator as a surrogate delay line, we demonstrate that fractional frequency variations at the level of 10^-12 are detectable, and that a Cs clock´s short-term Allan deviation can be measured without reference to another standard. The paper concludes with a discussion of how an ambiguity in the method might be resolved, and how the method might be employed in the optical domain.