Stray-field-induced quadrupole shift and absolute frequency of the 688-THz

We report experimental investigations of a single-ion optical frequency standard based on Yb+. The ion is confined in a cylindrically symmetric radiofrequency Paul trap. The reference transition is the S1/2 F=0 – D3/2 F =2 electric quadrupole transition at 688 THz. Using a differential measurement scheme, we determine the shift of the reference transition frequency that occurs due to the interaction of the electric quadrupole moment of the D3/2 state with the gradient of the electrostatic stray field in the trap. We determine an upper limit for the instability of the quadrupole shift over times between 100 s to 20 h. We also observe the variations in the shift and in the applied stray-field compensation voltages that result from loading a new ion into the trap and during a subsequent storage period of 74 days. This information is utilized to measure the absolute frequency of the reference transition with an uncertainty that is a factor of 3 smaller than that of the previous measurement. Using a fiber laser based optical frequency comb generator and the cesium fountain clock CSF1 of PTB Physikalisch-Technische Bundesanstalt , the frequency at 300 K temperature is determined as 688 358 979 309 306.62 0.73 Hz.