Al + 27 Quantum-Logic Clock with a Systematic Uncertainty below 10 − 18

We describe an optical atomic clock based on quantum-logic spectroscopy of the S01↔P30 transition in Al+27 with a systematic uncertainty of 9.4×10−19 and a frequency stability of 1.2×10−15/τ. A Mg+25 ion is simultaneously trapped with the Al+27 ion and used for sympathetic cooling and state readout. Improvements in a new trap have led to reduced secular motion heating, compared to previous Al+27 clocks, enabling clock operation with ion secular motion near the three-dimensional ground state. Operating the clock with a lower trap drive frequency has reduced excess micromotion compared to previous Al+27 clocks. Both of these improvements have led to a reduced time-dilation shift uncertainty. Other systematic uncertainties including those due to blackbody radiation and the second-order Zeeman effect have also been reduced.