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

Two Yb-171(+) 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 center dot 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 Yb-171(+) standard is measured with a relative systematic uncertainty of 3.1 center dot 10(-15) and a statistical uncertainty of 0.6 center dot 10(-15).