Optical frequency measurements are among the most precise tools available to science. With the rapid advances in optical clocks now achieving a low 10⁻¹⁷ stability at 1 s and averaging down to the 10⁻¹⁹ level in a few hundred seconds, real-time sensing of subtle phenomena becomes essential. To render possible such measurements, we introduce real-time optical phase tracking with ultra-low-noise frequency combs as a fundamental means to constantly monitor frequency offsets. This enables the characterization of optical frequency synthesis with stability and accuracy at the 20th decimal place within a measurement time of <100 s. To enable comb operation at this level of performance, dichroic heterodyne detection is used to compensate phase drifts occurring in the generation and dissemination of widely spaced optical frequencies. We qualify an example set-up by comparison with a reference system, measuring an offset between two combs of (5.4 ± 5.3) × 10⁻²¹ in one single measurement run of 1 × 10⁵ s.

光频率测量是可用于科学的最精确工具之一。随着光学时钟的飞速发展，现在在1 s内实现了低的10 ^-17稳定性，并且平均下降到10 ^-19在几百秒内保持水平，实时检测细微现象就变得至关重要。为了提供可能的测量结果，我们引入了具有超低噪声频率梳的实时光学相位跟踪，作为不断监测频率偏移的基本手段。这样就可以在小于100 s的测量时间内，在第20位小数点处以稳定和准确的方式表征光频率合成。为了在这种性能水平下实现梳状操作，二向色外差检测用于补偿在宽间隔的光学频率的产生和传播中出现的相位漂移。通过与参考系统进行比较，我们对示例设置进行了鉴定，以1×10 ^5的单次测量运行测量了（5.4±5.3）×10 ^−21的两个梳齿之间的偏移量 s。