Demonstration of 4.8 x10-17 stability at 1s for two independent optical clocks

Optical atomic clocks require local oscillators with exceptional optical coherence owing to the challenge of performing spectros-copy 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 x 10(-17) / root tau for an averaging time tau (in seconds). Synchronous interrogation enables each clock to average at a rate of 3.5 x 10(-17) / root tau, dominated by quantum projection noise, and reach an instability of 6.6 x 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.