We report sympathetic cooling of
113Cd
+ by laser-cooled
40Ca
+ in a linear Paul trap for microwave clocks. Long-term low-temperature confinement of
113Cd
+ ions was achieved. The temperature of these ions was measured at
90(10) mK, and the corresponding uncertainty arising from the second-order Doppler shifts was estimated to a level of
2×10−17. Up to
4.2×105 Cd
+ ions were confined in the trap, and the confinement time constant was measured to be 84 hours. After three hours of confinement, there were still
105 Cd
+ ions present, indicating that this Ca
+--Cd
+ dual ion system is surprisingly stable. The ac Stark shift was induced by the Ca
+ lasers and fluorescence, which was carefully estimated to an accuracy of
5.4(0.5)×10−17 using a high-accuracy \textit{ab initio} approach. The Dick-effect-limited Allan deviation was also deduced because deadtimes were shorter. These results indicate that a microwave clock based on this sympathetic cooling scheme holds promise in providing ultra-high frequency accuracy and stability.
This paper reports on trapping and laser-cooling of singly-ionized strontium ions in a linear Paul trap. We demonstrate loading of large ion clouds containing as much as 106 ions and laser cooling down to the Coulomb crystal transition. We observe the spatial segregation of the different Sr+ isotopes due to the mass-dependent Paul trap stiffness. Sympathetic cooling of the different isotopes is demonstrated, either by laser-cooling of 88Sr+ (83% abundance) or of 86Sr+ (10% abundance). These demonstrations open the way to the use of a large ion Coulomb crystal for quantum optics and quantum information experiments, where the strong confinement, the long lifetime, and the absence of perturbation by cooling lasers are crucial.