We demonstrate the long-distance transmission of an ultra-stable optical frequency derived directly from a state-of-the-art optical frequency standard. Using an active stabilization system we deliver the frequency via a 146 km long underground fiber link with a fractional instability of 3*10^{-15} at 1 s, which is close to the theoretical limit for our transfer experiment. The relative uncertainty for the transfer is below 1*10^{-19} after 30 000 seconds. Tests with a very short fiber show that noise in our stabilization system contributes fluctuations which are two orders of magnitude lower, namely 3*10^{-17} at 1 s, reaching 10^{-20} after 4000 s.
@article{arxiv.0904.2679,
title = {Optical frequency transfer via 146 km fiber link with 10^{-19} relative accuracy},
author = {G. Grosche and O. Terra and K. Predehl and R. Holzwarth and B. Lipphardt and F. Vogt and U. Sterr and H. Schnatz},
journal= {arXiv preprint arXiv:0904.2679},
year = {2016}
}