Cooling and Crystallization of Trapped ¹¹³Cd⁺ Ions for Atomic Clock

We report recent experimental progress toward a trapped ¹¹³Cd⁺ ion microwave frequency standard. With laser cooling, a large number of trapped cadmium ions are cooled to crystallization and the temperature of the ion crystal is measured to be 16 ± 3 mK. The second order Doppler frequency shift, a main limiting factor of the atomic clock's performance, is estimated to be less than 1 × 10^?16.