Transportable Clocks Move with the Times

Abstract

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, and an instability of 1.3 × 10 −15 = ffiffi ffi τ p with an averaging time τ in seconds. Measurements confirm that the systematic uncertainty can be reduced to below the design goal of 1 × 10 −17 . To our knowledge, these are the best uncertainties and instabilities reported for any transportable clock to date. For autonomous operation, the TOC has been installed in an airconditioned car trailer. It is suitable for chronometric leveling with submeter resolution as well as for intercontinental cross-linking of optical clocks, which is essential for a redefinition of the International System of Units (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 its development is an important step to space-borne optical clocks. DOI: 10.1103/PhysRevLett.118.073601 The best clocks reach fractional systematic uncertainties of a few 10 −18 [1-3] and instabilities near or even below 10 ,4-6], surpassing the clocks realizing the International System of Units (SI) second in both aspects by 2 orders of magnitude. These achievements have triggered a discussion about a redefinition of the SI second [7,8] and push the frontiers of precision spectroscopy [2,9,10] as well as tests of fundamental physics [10][11][12][13]. Further, these clocks enable chronometric leveling [14][15][16][17][18] with relevant resolution, where gravitational redshifts are exploited to measure height differences.So far, the operation of optical clocks has been constrained to laboratories. However, applications like chronometric leveling require transportable clocks to provide the necessary flexibility in the choice of measurement sites. Transportable optical clocks (TOCs) are highly interesting for frequency metrology and time keeping as well, enabling a consistent worldwide network of ultraprecise clocks. Although comparisons at the full performance level of state-of-the-art optical clocks are possible through a few specialized optical fiber links [19][20][21][22] on a continental scale [17,23], intercontinental links are restricted to satellite-based methods that do not reach the clock performance [24]. In contrast, a transfer standard enables world-wide interconnections between optical clocks exploiting their exquisitely small uncertainty and low instability. It will thus benefit the efforts towards a redefinition of the SI second.Making optical clocks compact and robust for transport is the first phase in granting a wide community of users access to these devices [25][26][27]. Furthermore, transportability is a relevant step towards applications of optical clocks in space. Although developments in these directions are ongoing [28][29][30][31], to our knowledge, the o...