171Yb+ single-ion optical frequency standard at 688 THz

Two ¹⁷¹Yb⁺ single-ion optical frequency standards operating at 688 THz (436 nm) are compared in order to investigate systematic frequency shifts in the sub-Hertz range. In the absence of externally applied perturbations, a mean relative frequency difference of 3.8(6.1)·10^-16 is observed. This agreement is comparable to that of the most accurate comparisons between cesium fountain clocks. Using a femtosecond frequency comb generator based on a Er³⁺-doped fiber laser, the frequency of the ¹⁷¹Yb⁺ standard is measured with a relative systematic uncertainty of 3.1·10^-15 and a statistical uncertainty of 0.6·10^-15.