2e-18 total uncertainty in an atomic clock

The pursuit of better atomic clocks has advanced many research areas, providing better quantum state control, new insights in quantum science, tighter limits on fundamental constant variation, and improved tests of relativity. We present an important step towards realizing the full potential of a many-particle clock with a state-of-the-art stable laser. Here, we achieve stability of 2.2e-16 at 1 s by using seconds-long coherent interrogations of our clock transition. With this better stability, we perform a new accuracy evaluation of our clock. For the lattice ac Stark systematic, we identify the lattice laser frequency where the scalar and tensor components of the shift cancel, allowing for state independent trapping with clock shifts at the 1e-18 level. For the BBR systematic, we improve our measurement of the atoms' thermal environment using accurate radiation thermometry traceable to the NIST ITS-90 absolute temperature scale. We also directly measure the component of the strontium atomic structure that is chiefly responsible for the spectral response to room-temperature BBR. Our combined measurements have reduced the total uncertainty of the JILA Sr clock to a record 2.1e-18.