171Yb+ Microwave Frequency Standard

Microwave frequency standards based on the 12.6 GHz ground state hyperfine transition in ¹⁷¹Yb⁺ have been under development at the National Measurement Institute, Australia, for many years. Using a laser-cooled ion cloud, the transition frequency was measured in 2001 to an accuracy of 8 parts in 10¹⁴ , limited by the homogeneity of the magnetic field. Uncertainties associated with field inhomogeneity in the new vacuum system are now below 1 part in 10¹⁵. We demonstrate that other systematic uncertainties such as AC Zeeman shift, microwave imperfections and pressure shifts permit operation in the 10^-15 accuracy range. The performance of the ¹⁷¹Yb⁺ microwave standard can therefore be comparable to that of a caesium fountain.