We developed a novel technique for frequency measurement and synthesis, based
on the operation of a femtosecond comb generator as transfer oscillator. The
technique can be used to measure frequency ratios of any optical signals
throughout the visible and near-infrared part of the spectrum. Relative
uncertainties of 10−18 for averaging times of 100 s are possible. Using a
Nd:YAG laser in combination with a nonlinear crystal we measured the frequency
ratio of the second harmonic νSH at 532 nm to the fundamental ν0 at
1064 nm, νSH/ν0=2.000000000000000001×(1±7×10−19).Comment: 4 pages, 4 figure