Lu+ optical frequency references with accuracy verified at the 19th digit

Optical atomic frequency references 1 have far surpassed their microwave frequency predecessors, leading to an anticipated redefinition of the SI (International System of Units) second 2 . However, as the scientific community seeks consensus on a new standard, and several state-of-the-art optical standards now report evaluated fractional uncertainties below 10 −18 , verification by same-species comparisons to comparable levels remains an outstanding challenge. Here we report two 176 Lu + single-ion optical frequency references, each with evaluated fractional frequency uncertainty near 1 × 10 −19 , which are directly compared by correlation spectroscopy and demonstrate agreement, with a measured relative frequency difference of [−0.1 ± (5.7) stat ± (1.0) sys ] × 10 −19 , where ‘stat’ and ‘sys’ indicate the statistical and systematic uncertainty, respectively.