Transportable Optical Lattice Clock with $7\times {10}^{-17}$ Uncertainty

We present a transportable optical clock (TOC) with ${}^{87}\mathrm{Sr}$. Its complete characterization against a stationary lattice clock resulted in a systematic uncertainty of $7.4\times {10}^{-17}$, which is currently limited by the statistics of the determination of the residual lattice light shift, and an instability of $1.3\times {10}^{-15}/\sqrt{\tau }$ with an averaging time $\tau$ in seconds. Measurements confirm that the systematic uncertainty can be reduced to below the design goal of $1\times {10}^{-17}$. 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 air-conditioned 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.

Figure 1

View into the car trailer for transport and operation. Front, left: Electronics for the laser systems. Back, left: Laser systems for cooling and trapping, and the reference cavities to frequency stabilize these lasers. Back center: Physics package. Front right: Computer control. Not shown are the interrogation and lattice laser setups. The interior dimensions of the container are $2.2\mathrm{m}\times 3\mathrm{m}\times 2.2\mathrm{m}$. The mass of the depicted experimental setup is approximately 800 kg. Inset: The car trailer from outside.

Figure 2

Sketch of the transportable reference cavity of the interrogation laser. The cavity is fixed to wires at ten points as indicated in red, green, and blue. These mounting points lie in the corresponding symmetry planes of the cavity to minimize vibration sensitivities. The wires are held by flexibly mounted bars to avoid stress on the cavity by mechanically overdetermined mounting. Each color-coded set of mounting elements restricts 1 rotational and 1 translational degree of freedom.

Figure 3

Total Allan deviation (totADEV) of the frequency ratio ${\nu }_{\text{trans}}/{\nu }_{\mathrm{stat}}$ (dots). The solid line indicates an instability of $1.3\times {10}^{-15}/\sqrt{\tau }$.

Figure 4

Section through the main vacuum chamber. The MOT coils wound from hollow, square wires with the coolant in the bore are located in reentrant flanges. The layout is indicated by the coolant flow and the color-coded coolant temperature. The windings exposed to the chamber are cooled first and are thus temperature-controlled best. The locations of some Pt100 temperature sensors are also indicated.