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.

由于在其超窄线宽时钟跃迁上执行光谱学的挑战，光学原子钟需要具有出色光学相干性的本地振荡器。因此，激光稳定技术的进步使时钟精度得以快速提高。基于低温硅参考腔的新型超稳定激光器最近证明了迄今为止最长的光学相干时间。在这里，我们利用具有两个锶（Sr）光学晶格时钟的本地振荡器来实现时钟稳定性的提高。通过反同步比较，在平均时间τ（以秒为单位）内，两个时钟的分数不稳定性被评估为\（4.8 \ times 10 ^ {-17} / \ sqrt \ tau \）。同步查询使每个时钟的平均速率为\（3.5 \ times 10 ^ {-17} / \ sqrt \ tau \），由量子投影噪声控制，在一小时的测量中达到6.6×10 ^-19的不稳定性。在如此短的时间内解决10 ^-18级以下频移的能力将影响量子感测和基础物理学中时钟的广泛应用。