We report an optical lattice clock with a total systematic uncertainty of $8.1\times {10}^{-19}$ in fractional frequency units, representing the lowest uncertainty of any clock to date. The clock relies on interrogating the ultranarrow ${{}^{1}S}_0\to {{}^{3}P}_0$ transition in a dilute ensemble of fermionic strontium atoms trapped in a vertically-oriented, shallow, one-dimensional optical lattice. Using imaging spectroscopy, we previously demonstrated record high atomic coherence time and measurement precision enabled by precise control of collisional shifts and the lattice light shift. In this work, we revise the black body radiation shift correction by evaluating the $5s4d$ ${}^3{D}_1$ lifetime, necessitating precise characterization and control of many body effects in the $5s4d$ ${}^3{D}_1$ decay. Last, we measure the second order Zeeman coefficient on the least magnetically sensitive clock transition. All other systematic effects have uncertainties below $1\times {10}^{-19}$.

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• Atomic, optical & lattice clocks