Development of a 171Yb+ microwave frequency standard at the National Measurement Institute, Australia

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 due to the stainless-steel vacuum chamber. We have designed and commissioned a new chamber in the alloy CrCu, which is non-magnetic and has good vacuum properties. The design incorporates large viewports and high-quality quartz windows for optical access. Uncertainties associated with field inhomogeneity in the new vacuum system are now below 1 part in 10¹⁵, a significant reduction which permits operation in the 10^-15 accuracy range. We have also demonstrated the use of photoionization to load the trap in a preliminary experiment, including isotope-selective loading