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Optical clock networks

Nature Photonics volume 11, pages 25–31 (2017)Cite this article

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Abstract

Within the last decade, optical atomic clocks have surpassed the best cesium clocks, which are used to realize the unit of time and frequency, in terms of accuracy and stability by about two orders of magnitude. When remote optical atomic clocks are connected by links without degradation in the clock signals, an optical clock network is formed, with distinct advantages for the dissemination of time, geodesy, astronomy and basic and applied research. Different approaches for time and frequency transfer in the microwave and optical regime, via satellites and free-space links, optical fibre links, or transportable optical atomic clocks, can be used to form a hybrid clock network that may allow a future redefinition of the unit of time based on an optical reference transition.

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Figure 1: Schematic of an optical atomic clock where optical transitions in atoms or ions are used to stabilize the frequency of a narrow-band laser to the centre of the transition.
Figure 2: Different methods can be used to compare the frequencies of optical atomic clocks.
Figure 3: Fractional instability of time and frequency transfer and clocks described by the Allan deviation (ADEV) or modified Allan deviation (mod ADEV) as function of the integration time τ.
Figure 4: Fibre links in Europe.

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Acknowledgements

The author thanks H. Schnatz, N. Huntemann, D. Piester, G. Grosche and U. Sterr for their valuable comments and suggestions.

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    Fritz Riehle

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Riehle, F. Optical clock networks. Nature Photon 11, 25–31 (2017). https://doi.org/10.1038/nphoton.2016.235

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