Since then, the accuracy of atomic clocks has improved dramatically, by a factor of 10 or so every decade. The best caesium standards have now reached an accuracy of one part in 1015, and of all the physical quantities, times and frequencies are the most precisely measured. Indeed, if the Earth's radius could be determined with a similar accuracy, then the measurement would be sensitive to the addition or removal of a few atoms on the planet's surface.
In the January issue of Physics World, Pierre Lemonde of BNM-LPTF, Observatoire de Paris, describes how the world's most accurate clock looks set to revolutionize international time standards and provide a testing ground for fundamental physics.