Cross-country time-keeping

RESEARCHERS have set a new record in the distance over which two atomic clocks can be synchronised via an optical fibre. Applications like GPS (Global Positioning System) navigation work by comparing timing signals coming from ultra-precise clocks at large distances. Such signals are normally relayed by satellite communication, but this cannot be done with the best available atomic clocks; they are so precise that small frequency shifts occurring during satellite transmission would be larger than the clock’s precision. Past experiments have shown that optical fibre links on the ground can replace satellites, by faithfully transmitting clock frequencies over hundreds of kilometres. Now, researchers at the Max-Planck-Institute of Quantum Optics in Germany have doubled the previous distance record, connecting two clocks over a distance of 1,840 km via a dedicated optical fibre between two German research centres. The method may one day link together optical clocks around the world. The work has been published in the latest issue of Physical Review Letters.