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

We describe an optical atomic clock based on quantum-logic spectroscopy of the ¹S₀ ↔³P₀ transition in ²⁷Al⁺ with a systematic uncertainty of 9.4 ×10^-19 and a frequency stability of 1.2 ×10^-15/√{τ }. A <SU>25Mg⁺ ion is simultaneously trapped with the ²⁷Al⁺ ion and used for sympathetic cooling and state readout. Improvements in a new trap have led to reduced secular motion heating, compared to previous ²⁷Al⁺ 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 ²⁷Al⁺ 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.