Toward a high-performance transportable microwave frequency standard based on sympathetically cooled 113Cd+ ions

We report a high-performance transportable microwave frequency standard scheme based on sympathetically cooled 113Cd+ ions using 40Ca+ as coolant ions. A transportable physical package (1.4 m3) with high-performance magnetic shielding (radial shielding coefficient 460 000) has been built. In experiments, more than 105 113Cd+ ions were sympathetically cooled to 90 mK in a Paul trap and the ion-loss time constant extends to 84 hours. Without direct cooling procedures, the Dick effect-limited Allan deviation due to the deadtimes is reduced to 1.8 × 10 − 14 / τ. Under our scheme, the uncertainties in the main frequency shifts such as those from second-order Zeeman and second-order Doppler terms were reduced below the order of 10 − 15. The relevant ac Stark shifts uncertainties were estimated to be below 5 × 10 − 18. Minor shifts further reflect the superiority of coolant ions 40Ca+ for 113Cd+. These results indicate that a microwave clock based on this sympathetic cooling scheme shows potential in establishing a vehicle transportable ground-based time-frequency reference for navigation systems.