Remote comparison of two Sr optical lattice clocks through a 58 km fiber link

Two ⁸⁷Sr optical lattice clocks (OLCs) are being developed and operated at the National Institute of Metrology (NIM), located on different campuses that are 40 km apart. In order to compare the optical frequencies of these two Sr OLCs, a 58-km noise canceled fiber link is built to transfer both a 1542-nm transfer laser and a microwave reference from Changping campus to Hepingli campus. Two commercial optical frequency combs (OFCs) with adapted single-branch 698/1542 nm outputs coherently unite the two 698-nm clock lasers and the 1542-nm transfer laser. The fractional instability of the comparison yields 3.1×10^-17 at 10000 s averaging time. The measured fractional frequency difference between these two Sr OLCs evaluated to be 1.9 (3.2)×10^-17 is within their claimed uncertainties, which shows the consistency of the frequencies of these two Sr OLCs when serving as optical frequency standards. Our remote comparison demonstrates the feasibility of optical clock comparison through long-distance fiber link and contributes to the progress of redefinition of the SI second.