Spin squeezing-enhanced clock comparison at the 10^-18 level

State-of-the-art optical clock comparisons are approaching a fundamental precision limit due to the quantum fluctuations of uncorrelated particles, the Standard Quantum Limit (SQL). Entanglement, in the form of spin squeezing, offers an avenue to bypass the SQL by redistributing the spin uncertainty to allow for more precise state readout. Realizing quantum advantage from spin squeezing in a state-of-the-art optical clock, however, remains challenging. Here we demonstrate a sub-SQL clock comparison, realizing a single-ensemble fractional frequency stability of 1.1x10^-18. We utilize cavity-based quantum nondemolition (QND) measurements to prepare two independent ensembles of ~30,000 atoms, and achieve a synchronous clock comparison with metrological improvement of 2.0(2) dB beyond the SQL, after correcting for state preparation and measurement errors. These results establish the most precise entanglement-enhanced clock to date and offer a powerful platform for exploring the interplay of gravity and quantum entanglement.