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.

Full article:

Nature Photonics (2019)

doi:10.1038/s41566-019-0520-5