Frequency equilibration in the vapor-cell atomic clock

Over the past several years, anecdotal evidence has grown indicating that Rb vapor-cell frequency standards exhibit a long "frequency equilibration" period following activation (i.e., time constant ∼10² days). Though this long equilibration period can have important implications for diverse timekeeping systems, the mechanism driving the behavior is not well understood and has been the subject of debate. In this paper, we investigate this phenomenon for a number of Rb vapor-cell clocks with the purpose of 1) establishing the validity of frequency equilibration as a generic vapor-cell atomic clock phenomenon and 2) obtaining characteristics of the equilibration process that can be used to constrain hypothesized frequency-equilibration mechanisms. Consistent with anecdotal evidence, results on three separate manufacturers\´ clocks indicate that frequency equilibration is a generic vapor-cell clock phenomenon, observable in both laboratory-based and space-based systems. However, the experimental data also casts doubt on the two mechanisms most often offered to explain frequency equilibration: helium permeation through the resonance cell\´s glass envelope, and the intensity-dependent light-shift effect. To help guide further research, we propose two alternate mechanisms for frequency equilibration: alkali surface diffusion on the resonance-cell and lamp walls, and the spectrum-dependent light-shift effect.