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.

We have measured the absolute frequency of the optical lattice clock based on 87 Sr at PTB with an uncertainty of 3.9 10 16 × − using two caesium fountain clocks. This is close to the accuracy of todayʼs best realizations of the SI second. The absolute frequency of the 5 s 2 1 S 0 -5s5p 3 P 0 transition in 87 Sr is 429 228 004 229 873.13(17) Hz. Our result is in excellent agreement with recent measurements performed in different laboratories worldwide. We improved the total systematic uncertainty of our Sr frequency standard by a factor of five and reach 3 10 17 × − , opening new prospects for frequency ratio measurements between optical clocks for fundamental research, geodesy or optical clock evaluation.

Optical clocks are not only powerful tools for prime fundamental research, but are also deemed for the re-definition of the SI base unit second as they now surpass the performance of caesium atomic clocks in both accuracy and stability by more than an order of magnitude. However, an important obstacle in this transition has so far been the limited reliability of the optical clocks that made a continuous realization of a timescale impractical. In this paper, we demonstrate how this situation can be resolved and that a timescale based on an optical clock can be established that is superior to one based on even the best caesium fountain clocks. The paper also gives further proof of the international consistency of strontium lattice clocks on the 10 −16 accuracy level, which is another prerequisite for a change in the definition of the second.

We present a transportable optical clock (TOC) with 87 Sr. Its complete characterization against a stationary lattice clock resulted in a systematic uncertainty of 7.4 × 10 −17 which is currently limited by the statistics of the determination of the residual lattice light shift. The measurements confirm that the systematic uncertainty is reduceable to below the design goal of 1 × 10 −17 . The instability of our TOC is 1.3 × 10 −15 / √ τ . Both, the systematic uncertainty and the instability are to our best knowledge currently the best achieved with any type of transportable clock. For autonomous operation the TOC is installed in an air-conditioned car-trailer. It is suitable for chronometric leveling with sub-meter resolution as well as intercontinental cross-linking of optical clocks, which is essential for a redefiniton of the SI second. In addition, the TOC will be used for high precision experiments for fundamental science that are commonly tied to precise frequency measurements and it is a first step to space borne optical clocks. PACS numbers:The best clocks in the world reach a fractional systematic uncertainty at the low 10 −18 level [1-4] and instabilities near or even below 10 2,[4][5][6], surpassing the clocks realizing the SI second in both by two orders of magnitude. This has triggered a discussion about a redefinition of the SI second [7,8], pushes the frontiers of precision spectroscopy and tests fundamental physics [9][10][11][12][13][14], and enables chronometric leveling [15][16][17][18][19], where gravitational redshifts are exploited to measure height differences.So far, the operation of optical clocks has been constrained to laboratories. However, transportable clocks are required for the necessary flexibility in the choice of measurement sites for applications like chronometric leveling. Also, they are highly interesting for frequency metrology and time keeping in creating a consistent worldwide network of the next-generation ultraprecise clocks. Although comparisons at the full performance level of state-of-the-art optical clocks are possible on a continental scale [18,19] through a few specialized optical fiber links [20][21][22], intercontinental links are so far restricted to satellite-based methods that cannot fully exploit the clock performance [23]. A transfer standard enables world-wide interconnections between optical clocks and will thus benefit the efforts towards a redefinition of the SI second.Making laboratory setups compact and robust for transport is also the first step towards granting a wide community of users access to these devices [24][25][26]. Furthermore, transportability is a first step towards applications of optical clocks in space. Developments in these directions are ongoing for optical lattice clocks (OLCs) with strontium [27,28]; however, to our knowledge the single-ion clock reported recently [29] is the only other transportable clock with uncertainty below 10 −16 .The requirements on such a TOC are challenging indeed: To enable comparisons of other optical cl...

Phase compensated optical fiber links enable high accuracy atomic clocks separated by thousands of kilometers to be compared with unprecedented statistical resolution. By searching for a daily variation of the frequency difference between four strontium optical lattice clocks in different locations throughout Europe connected by such links, we improve upon previous tests of time dilation predicted by special relativity. We obtain a constraint on the Robertson-Mansouri-Sexl parameter |α| 1.1 × 10 −8 quantifying a violation of time dilation, thus improving by a factor of around two the best known constraint obtained with Ives-Stilwell type experiments, and by two orders of magnitude the best constraint obtained by comparing atomic clocks. This work is the first of a new generation of tests of fundamental physics using optical clocks and fiber links. As clocks improve, and as fiber links are routinely operated, we expect that the tests initiated in this paper will improve by orders of magnitude in the near future.