Abstract
Leveraging the unrivalled performance of optical clocks as key tools for geo-science, for astronomy and for fundamental physics beyond the standard model requires comparing the frequency of distant optical clocks faithfully. Here, we report on the comparison and agreement of two strontium optical clocks at an uncertainty of 5 × 10−17 via a newly established phase-coherent frequency link connecting Paris and Braunschweig using 1,415 km of telecom fibre. The remote comparison is limited only by the instability and uncertainty of the strontium lattice clocks themselves, with negligible contributions from the optical frequency transfer. A fractional precision of 3 × 10−17 is reached after only 1,000 s averaging time, which is already 10 times better and more than four orders of magnitude faster than any previous long-distance clock comparison. The capability of performing high resolution international clock comparisons paves the way for a redefinition of the unit of time and an all-optical dissemination of the SI-second.
Highlights
Leveraging the unrivalled performance of optical clocks as key tools for geo-science, for astronomy and for fundamental physics beyond the standard model requires comparing the frequency of distant optical clocks faithfully
The capabilities of distant optical clocks could so far neither be fully tested nor exploited. We overcome these limitations by a direct, all-optical frequency comparison between two optical clocks via a telecom fibre link and demonstrate a remote optical clock comparison with one order of magnitude better accuracy and orders of magnitude shorter averaging time than with microwave clocks or satellite links
Our work enables the above mentioned applications, and clears the path towards a redefinition of the unit of time, the SI second[24,25] through regular and practical international comparisons of optical clocks. They are of utmost importance for an evolution of the international time scale temps atomique international (TAI), which delivers the accuracy of primary frequency standards to a wide range of users in science and society
Summary
Leveraging the unrivalled performance of optical clocks as key tools for geo-science, for astronomy and for fundamental physics beyond the standard model requires comparing the frequency of distant optical clocks faithfully. The traditional means to compare remote optical clocks are using microwave signals: the optical clocks’ frequencies are measured against primary caesium microwave clocks and the frequency values measured locally are compared, or satellitebased methods transferring the frequency are applied[20] The accuracy of the former method is limited by the caesium microwave clocks with the most stringent bound set by our groups[5,21,22,23] at an agreement within 4 Â 10 À 16; satellite link techniques do not reach significantly better performance either. They are of utmost importance for an evolution of the international time scale temps atomique international (TAI), which delivers the accuracy of primary frequency standards to a wide range of users in science and society
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