ABSTRACT Microwave frequency standards based on buffer gas-cooled 171Yb+ ions have demonstrated high stability but are limited in accuracy by the second-order Doppler shift caused by thermal motion. We have previously obtained near shot noise-limited Ramsey fringes with a laser-cooled ion cloud. Here, we present measurements confirming that the ion temperature remains <1 K throughout the microwave interrogation period for a Ramsey pulse separation of up to 10 s and longer. The potential stability of the ions as a frequency standard is better than σy(τ)=5× 10-14τ-1/2, and estimates of the systematic offsets to the clock frequency and their uncertainties indicate that a total uncertainty of four parts in 1015 or better is achievable.
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