  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. The record for the best stability and accuracy is currently held by
optical lattice clocks. This work takes an important step towards realizing the
full potential of a many-particle clock with a state-of-the-art stable laser.
Our 87Sr optical lattice clock now achieves fractional stability of 2.2e-16 at
1 s. With this improved stability, we perform a new accuracy evaluation of our
clock, reducing many systematic uncertainties that limited our previous
measurements, such as those in the lattice ac Stark shift, the atoms' thermal
environment, and the atomic response to room-temperature BBR. Our combined
measurements have reduced the total uncertainty of the JILA Sr clock to 2.1e-18
in fractional frequency units.
