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...

With 87Sr atoms confined in a one-dimensional optical lattice, the frequency of the optical clock transition 5s2 1S0–5s5p 3P0 has been determined to be 429 228 004 229 872.9(5) Hz. The transition frequency was measured with the help of a femtosecond-frequency comb against one of Physikalisch-Technische Bundesanstalt (PTB's) H-masers whose frequency was measured simultaneously by the PTB Cs-fountain clock CSF1. The Sr optical frequency standard contributes with a fractional uncertainty of 1.5 × 10−16 to the total uncertainty. The agreement of the measured transition frequency with previous measurements at other institutes supports the status of this transition as the secondary representation of the second with the currently smallest uncertainty.

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