¹⁷¹Yb⁺ Single-Ion Optical Frequency Standard at 688 THz

Two171Yb+single-ion optical frequency standards operating at 688 THz (436 nm) are compared in order to investigate systematic frequency shifts in the subhertz range. In the absence of externally applied perturbations, a mean relative frequency difference of 3.8 10-16is observed. Using a femtosecond frequency comb generator based on an Er3+-doped fiber laser, the frequency of the171Yb+standard is measured with a relative systematic uncertainty of 3.1 10-15and a statistical uncertainty of 0.6 - 10-15