High-stability optical clock based on a continuously ground-state cooled Al + ion without compromising its accuracy

Single-ion optical clocks have shown systematic frequency uncertainties below 10

⁻¹⁸, but typically require more than one week of averaging to achieve a corresponding statistical uncertainty. This time can be reduced with longer probe times, but comes at the cost of a higher time-dilation shift due to motional heating of the ions in the trap. We show that sympathetic ground-state cooling using electromagnetically induced transparency of an Al

⁺ clock ion via a cotrapped Ca

⁺ ion during clock interrogation suppresses the heating of the ions. Al

⁺ can be kept close to the motional ground state, independent from the chosen interrogation time, at a relative time-dilation shift of (−1.69±0.20)×10

⁻¹⁸. The Ca

⁺ cooling light introduces an additional light shift on the Al

⁺ clock transition of (−9.3±1.1)×10

⁻¹⁸. We project that the uncertainty of this light shift can be further reduced by nearly an order of magnitude. This sympathetic cooling enables seconds of interrogation time with 10

⁻¹⁹ motional and cooling laser-induced uncertainties for Al

⁺ and can be employed in other ion clocks as well.