Coherent Transfer of Optical Frequency over 112km with Instability at the 10^-20 Level

We demonstrate optical-carrier transfer over a 112-km single-span urban fiber link. By actively compensating the phase noise induced along the fiber link, a noise suppression of 55 dB at 1 Hz is obtained. A fractional frequency instability of 2.5\times10^-16 at 1 s is achieved, and reaching 7.5\times10^-20 at 10000 s. The system is stable and able to run for a long time. This work will contribute to optical frequency distribution and remote comparison among atomic clocks.