In August 2016 the French LNE-SYRTE in Paris and German PTB in Braunschweig reported the comparison and agreement of two fully independent experimental strontium lattice optical clocks in Paris and Braunschweig at an uncertainty of 5 × 10 −17 via a newly established phase-coherent frequency link connecting Paris and Braunschweig, using 1415 kilometers of telecom fibre-optic cable.