Liquid-Nitrogen-Cooled <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msup> <mml:mo></mml:mo> <mml:mn>40</mml:mn> </mml:msup> <mml:msup> <mml:mi>Ca</mml:mi> <mml:mo>+</mml:mo> </mml:msup> </mml:math> Ion Optical Clock with a Systematic Uncertainty of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mn>4.4</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>19</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math>

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.