Sympathetic cooling of 113Cd+ by laser-cooled 40Ca+ in a linear Paul trap for Microwave Ion Clocks
Abstract
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 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.