$^{87}$Sr Optical Lattice Clock Comparison with 6$\times 10^{-19}$ Precision in 1 Hour

Utilizing a next generation ultrastable laser based on a cryogenic silicon cavity we perform an extensive comparison between JILA’s 1D and 3D strontium clocks, achieving record independent clock stability of 4.8$\times 10^{-17}$ at 1 s. Through synchronous measurement we determine record clock stability of 3.5$\times 10^{-17}$ at 1 s and average to a precision of 6$\times 10^{-19}$ in 1 hour of measurement. This state-of-the-art clock precision enables measurements for wide-ranging applications, from searches for dark matter to relativistic geodesy. Additionally, the combined accuracy of our fully evaluated 1D $^{87}$Sr clock with the long-term stability of the silicon cavity paves a potential path to the realization of an all optical timescale, which promises to outperform current microwave timescales.