(176)Lu(+) clock comparison at the 10(-18) level via correlation spectroscopy.

The extreme precision of optical atomic clocks has led to an anticipated redefinition of the second by the International System of Units. Furthermore, accuracies pushing the boundary of 1 part in 10(18) and beyond will enable new applications, such as in geodesy and tests of fundamental physics. The (1)S(0) to (3)D(1) optical transition in (176)Lu(+) has exceptionally low sensitivity to external perturbations, making it suitable for practical clock implementations with inaccuracy at or below 10(-18). Here, we perform high-accuracy comparisons between two (176)Lu(+) references using correlation spectroscopy. A comparison at different magnetic fields is used to obtain a quadratic Zeeman coefficient of -4.89264(88) Hz/mT for the reference frequency. With a subsequent comparison at low field, we demonstrate agreement at the low 10(-18) level, statistically limited by the averaging time of 42 hours. The evaluated uncertainty in the frequency difference is 9 × 10(-19) and the lowest reported in comparing independent optical references.