Liquid-Nitrogen-Cooled ​^{40}Ca^{+} Ion Optical Clock with a Systematic Uncertainty of 4.4×10^{-19}

We report a single-ion optical clock based on the 4S_{1/2}→3D_{5/2} transition of the ^{40}Ca^{+} ion, operated in a liquid nitrogen cryogenic environment, achieving a total systematic uncertainty of 4.4×10^{-19}. We employ a refined temperature evaluation scheme to reduce the frequency uncertainty due to blackbody radiation, and 3D sideband cooling to minimize the second-order Doppler shift. We have precisely determined the average Zeeman coefficient of the ^{40}Ca^{+} clock transition to be 14.345(15)  Hz/mT^{2}, thereby significantly reducing the quadratic Zeeman shift uncertainty. Moreover, the cryogenic environment enables the lowest reported heating rate due to ambient electric field noise in trapped-ion optical clocks.