An ²⁷Al⁺ quantum-logic clock with 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 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.