$^{171}\hbox{Yb}^{+}$ Single-Ion Optical Frequency Standard at 688 THz

Two
¹⁷¹
Yb
⁺
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
^-16
is observed. Using a femtosecond frequency comb generator based on an Er
³⁺
-doped fiber laser, the frequency of the
¹⁷¹
Yb
⁺
standard is measured with a relative systematic uncertainty of 3.1 10
^-15
and a statistical uncertainty of 0.6 - 10
^-15