Physics

Laser linewidth

Instability

Laser

Optics

Atomic physics

Atomic clock

Superfluidity

Coherence (philosophical gambling strategy)

Quantum mechanics

Optical lattice

Nature Photonics

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.

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