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

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 × 10−15in 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 μrad 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. The accuracy of the associated systematic effect is evaluated at 3 × 10−15.