Absolute frequency measurements with a robust, transportable ⁴⁰Ca⁺ optical clock

We constructed a transportable ⁴⁰Ca⁺ optical clock (with an estimated minimum systematic shift uncertainty of 1.3 x 10^-17 and a stability of 5 x 10^-15/√τ) that can operate outside the laboratory. We transported it from the Innovation Academy for Precision Measurement Science and Technology, Chinese Academy of Sciences, Wuhan to the National Institute of Metrology, Beijing. The absolute frequency of the 729 nm clock transition was measured for up to 35 d by tracing its frequency to the second of International System of Units. Some improvements were implemented in the measurement process, such as the increased effective up-time of 91.3% of the ⁴⁰Ca⁺ optical clock over a 35-day-period, the reduced statistical uncertainty of the comparison between the optical clock and hydrogen maser, and the use of longer measurement times to reduce the uncertainty of the frequency traceability link. The absolute frequency measurement of the ⁴⁰Ca⁺ optical clock yielded a value of 411 042 129 776 400.26 (13) Hz with an uncertainty of 3.2 x 10^-16, which is reduced by a factor of 1.7 compared with our previous results. As a result of the increase in the operating rate of the optical clock, the accuracy of 35 d of absolute frequency measurement can be comparable to the best results of different institutions in the world based on different optical frequency measurements. (author)