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.