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Real-time phase tracking for wide-band optical frequency measurements at the 20th decimal place

Abstract

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.

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Fig. 1: Schematic of dichroic heterodyne detection for phase drift compensation.
Fig. 2: Experimental set-up.
Fig. 3: Phase noise and phase tracking of the uncompensated heterodyne beat notes.
Fig. 4: Phase tracking and fractional frequency instability employing dichroic heterodyne detection.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

We acknowledge funding from the European Union’s 7th Framework Programme (EU FP7) Marie Skłodowska-Curie Initial Training Network Future Atomic Clock Technology (FACT), the Defense Advanced Research Projects Agency’s (DARPA) Program in Ultrafast Laser Science and Engineering (PULSE, PμreComb project) under contract no. W31P4Q-14-C-0050 and the German Space Agency (DLR) projects ‘Faser-optischer Kammgenerator für angewandte LIDAR-Spektroskopie’ (FOKAL), ‘Faser-optischer Kammgenerator unter Schwerelosigkeit’ (FOKUS and FOKUS II) and ‘InfraRed Astronomy Satellite Swarm Interferometry’ (IRASSI). We thank H. Katori and J. Ye for insightful discussions and colleagues from Menlo Systems for technical support.

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M.G. and W.H. performed the experiments, conceived and realized the two frequency combs and the optical set-up. M.G. analysed the data. M.G. and W.H. wrote the manuscript. M.F. and M.L. managed the project. T.U. provided deep insight into the interpretation of the results and aided with the optical set-up. R.H. initiated and led the activities. All co-authors commented on and improved the manuscript.

Corresponding author

Correspondence to Michele Giunta.

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Giunta, M., Hänsel, W., Fischer, M. et al. Real-time phase tracking for wide-band optical frequency measurements at the 20th decimal place. Nat. Photonics 14, 44–49 (2020). https://doi.org/10.1038/s41566-019-0520-5

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