Demonstration of the nearly continuous operation of an 171Yb optical lattice clock for half a year

Optical lattice clocks surpass primary Cs microwave clocks in frequency stability and accuracy, and are promising candidates for a redefinition of the second in the International System of Units (SI). However, the robustness of optical lattice clocks has not yet reached a level comparable to that of Cs fountain clocks which contribute to International Atomic Time (TAI) by the nearly continuous operation. In this paper, we report the long-term operation of an Yb-171 optical lattice clock with a coverage of 80.3% for half a year including uptimes of 93.9% for the first 24 days and 92.6% for the last 35 days. This enables a nearly dead-time-free frequency comparison of the optical lattice clock with TAI over months, which provides a link to the SI second with an uncertainty of low 10(-16). By using this link, the absolute frequency of the (1)S0-3<iP(0) clock transition of Yb-171 is measured as 518 295 836 590 863.54(26) Hz with a fractional uncertainty of 5.0 x 10(-16). This value is in agreement with the recommended frequency of Yb-171 as a secondary representation of the second.