Abstract
We report a single-ion optical atomic clock with fractional frequency uncertainty of 5.5×10⁻¹⁹ and fractional frequency stability of 3.5 ×10⁻¹⁶/√τ/s, based on quantum logic spectroscopy of a single ²⁷Al⁺ ion. A co-trapped ²⁵Mg⁺ ion provides sympathetic cooling and quantum logic readout of the ²⁷Al⁺ ¹S₀↔³P₀ clock transition. A Rabi probe duration of 1 s, enabled by laser stability transfer from a remote cryogenic silicon cavity across a 3.6 km fiber link, results in a threefold reduction in instability compared to previous ²⁷Al⁺ clocks. Systematic uncertainties are lower due to an improved ion trap electrical design, which reduces excess micromotion, and a new vacuum system, which reduces collisional shifts. We also perform a direction-sensitive measurement of the ac magnetic field due to the RF ion trap, eliminating systematic uncertainty due to field orientation.