We report a single-ion optical atomic clock with a fractional frequency uncertainty of $5.5\times {10}^{-19}$ and fractional frequency stability of $3.5\times {10}^{-16}/\sqrt{\tau /\mathrm{s}}$, based on quantum logic spectroscopy of a single ${}^{27}{\mathrm{Al}}^{+}$ ion. A cotrapped ${}^{25}{\mathrm{Mg}}^{+}$ ion provides sympathetic cooling and quantum logic readout of the ${}^{27}{\mathrm{Al}}^{+}{{}^{1}S}_0\leftrightarrow {{}^{3}P}_0$ 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 ${}^{27}{\mathrm{Al}}^{+}$ 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.

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• Atomic, optical & lattice clocks
• Quantum nondemolition measurement
• Trapped ions
• Polarizability
• Atom & ion cooling