A multi-ion optical clock with 5.3×10^-19 systematic uncertainty

Scaling trapped-ion optical clocks to multiple ions promises reduced measurement times, but poses the challenge of controlling frequency shifts across the Coulomb crystal. We present a new optical clock with 8 to 10 ⁸⁸Sr⁺ ions and a fractional systematic uncertainty of 5.3×10^-19.  A linear Coulomb crystal of ⁸⁸Sr⁺ ions is collectively addressed: For Doppler cooling and state preparation we use elliptical laser beams, while interrogation of the ²S_1/2 - ²D_5/2 clock transition is performed with a beam propagating along the ion chain. We obtain ion-resolved state detection by implementing camera-based fluorescence observations into our experimental control sequence. This allows us to investigate and control frequency shifts that vary for different ion positions in the chain. 

An optical clock comparison between the new system and an established single-ion clock based on the ²S_1/2 - ²F_7/2 transition in ¹⁷¹Yb⁺ yields consistent results between the Sr⁺ clock operating with either a single or up to 10 ions. Operation with multiple ions significantly improves the measurement instability. For the frequency ratio, we achieve a statistical uncertainty of 0.9×10^-18 and a systematic uncertainty of 2.8×10^-18, dominated by that of the Yb⁺ clock.