Comparison of two ¹⁷⁶Lu⁺ optical frequency references having 1x10⁻¹⁹ systematic uncertainty with 5.3 × 10^-19 precision 

The ¹⁷⁶Lu^{+ 1}S₀-³D₁ optical frequency standard is insensitive to electromagnetic shifts to the extent that it can readily achieve uncertainties below 10^-18 when operated at room temperature without thermal or magnetic shielding.  This has allowed us to construct two single-ion optical references each with a fractional frequency uncertainty near 1 x 10^-19, which is several times more accurate than the best reported references to date.

We support our accuracy claim by direct comparison of two independent systems via correlation spectroscopy, demonstrating frequency agreement of [-2.4 ± (5.3)_stat ± (1.0)_sys] x 10^-19 where stat and sys indicate the statistical and systematic uncertainties respectively. This is the first report to demonstrate agreement of frequency standards below 10^-18.  We discuss the developments that have made this result possible, and the problems faced with the frequency metrology community in the future application of optical standards and redefinition of the second.