Long-Distance Frequency Dissemination with a Resolution of10−17

We use a new technique to disseminate microwave reference signals along ordinary optical fiber. The fractional frequency resolution of a link of 86 km in length is ${10}^{-17}$ for a one day integration time, a resolution higher than the stability of the best microwave or optical clocks. We use the link to compare the microwave reference and a ${\mathrm{CO}}_2/{\mathrm{OsO}}_4$ frequency standard that stabilizes a femtosecond laser frequency comb. This demonstrates a resolution of $3\times {10}^{-14}$ at 1 s. An upper value of the instability introduced by the femtosecond laser-based synthesizer is estimated as $1\times {10}^{-14}$ at 1 s.