A clock with 8×10^-19 systematic uncertainty

We report an optical lattice clock with a total systematic uncertainty of 8.1 × 10^-19 in fractional frequency units, representing the lowest uncertainty of any clock to date. The clock relies on interrogating the ultra-narrow ^1S_0 →^3P_0 transition in a dilute ensemble of fermionic strontium atoms trapped in a vertically-oriented, shallow, one-dimensional optical lattice. With 10^5 atoms in Wannier-Stark eigenstates of this lattice, we measure record atomic coherence time and measurement precision reaching below 1 × 10^-19 [arXiv:2109.12238]. Such clock precision, together with imaging spectroscopy, enables precise control of collisional shifts as well as the lattice light shift [arXiv:2201.05909, arXiv:2210.16374]. To address two remaining large systematic effects, we measure the second order Zeeman coefficient on the least magnetically sensitive clock transition, and we precisely determine the 5s4d ^3D_1 lifetime to reduce the dynamic black body radiation shift uncertainty. All other systematic effects have uncertainties below 1 × 10^-19.