a Microwave Frequency Standard Based on Laser-Cooled 171Yb+ Ions

We report the first operation of a microwave frequency standard based on the 12.6 GHz ground-state hyperfine transition in a cloud of laser-cooled ¹⁷¹Yb⁺ ions, with a preliminary measurement of the transition frequency. We obtain 12 642 812 118.468 5(7)(6) Hz, where the first uncertainty combines statistical uncertainty and the uncertainty of the systematic shifts in the transition frequency due to the trap environment, and the second is that of a comparison between a reference hydrogen maser and the SI second. This value is in agreement with earlier measurements obtained for buffer gas-cooled ions where the second-order Doppler shift is 180 times larger. A full evaluation of systematic shifts is still pending, but the dominant effects are discussed. Motional second-order Doppler shifts are determined within an uncertainty of 1-2 parts in 10¹⁵ from measurements of the ion temperature. Our progress to date indicates no serious obstacle to realising a combined fractional frequency uncertainty of 4 × 10^-15 or smaller and a projected stability of approximately 5 × 10^-14τ^-1/2 for a 10 s Ramsey time. The performance of the ¹⁷¹Yb⁺ microwave standard can therefore be comparable to that of a cesium fountain.