Demonstration of 4.8 × 10−17 stability at 1 s for two independent optical clocks

Optical atomic clocks require local oscillators with exceptional optical coherence owing to the challenge of performing spectroscopy on their ultranarrow-linewidth clock transitions. Advances in laser stabilization have thus enabled rapid progress in clock precision. A new class of ultrastable lasers based on cryogenic silicon reference cavities has recently demonstrated the longest optical coherence times to date. Here we utilize such a local oscillator with two strontium (Sr) optical lattice clocks to achieve an advance in clock stability. Through an anti-synchronous comparison, the fractional instability of both clocks is assessed to be $4.8 \times 10^{ - 17}/\sqrt \tau for an averaging time τ (in seconds). Synchronous interrogation enables each clock to average at a rate of 3.5 \times 10^{ - 17}/\sqrt \tau , dominated by quantum projection noise, and reach an instability of 6.6 × 10−19 over an hour-long measurement. The ability to resolve sub-10−18-level frequency shifts in such short timescales will affect a wide range of applications for clocks in quantum sensing and fundamental physics. By using an ultrastable oscillator based on a cryogenic Si cavity, the fractional instability of two Sr optical lattice clocks at 1 s reaches 4.8 × 10−17 and 3.5 × 10−17 through anti-synchronous and synchronous comparisons, respectively.