A 920-Kilometer Optical Fiber Link for Frequency Metrology at the 19th Decimal Place

Abstract

Synchronize Watches: Time standards based on the energy-level transitions of atoms and ions provide the most accurate and precise methods of time keeping. Measurements made in one laboratory and in another must be done with clocks that have been synchronized and calibrated to ensure that the same measurement is being made. Such clocks, however, are not particularly mobile and are housed in national metrology labs. Predehl et al. (p. 441; see the Perspective by Warrington ) linked two optical clocks separated by over 900 kilometers using optical fiber to show that the clocks can be synchronized, with the clocks showing a frequency stability better than 3.7 × 10 ⁻¹⁹ . Such long-distance synchronization should allow for tests of fundamental physics, such as general relativity and quantum electrodynamics.