Development of a Phase Transient Test Bench to Optimize a Cold-Atom Microwave Clock

Abstract

MuClock is a commercial transportable microwave clock developed at Exail Quantum Systems, using rubidium atoms cooled by isotropic laser light. MuClock is comparable with typical hydrogen masers in terms of volume and long-term frequency stability, as it nominally reaches a fractional frequency stability of $1\times 10^{-15}$ in less than two days and maintains this level of performance over more than one month of integration time. The evaluation of several systematic frequency shifts is on-going. This article presents the impact of the frequency bias induced by microwave phase transients on the clock frequency stability and accuracy. Using an in-house designed test bench, the phase of the 6.8 GHz microwave clock signal is measured with a phase resolution close to 1 $\mu $ rad and a temporal resolution below 100 ns. It enables a thorough optimization of the sequence and a better control of each phase transient event, making their contributions to the overall frequency shift negligible. Phase peaks induced by electronic components operating in parallel are shifted in time so that their contributions become negligible compared with other frequency uncertainties. The accuracy of the associated systematic effect is evaluated at $3\times 10^{-15}$ .