Caesium atomic clocks: function, performance and applications

For more than four decades, caesium atomic clocks have been the backbone in a variety of demanding applications in science and technology. Neither satellite based navigation systems, like the US Global Positioning System, nor the syntonization of telecommunication networks at the presently prescribed levels, would function without them. Recent years have brought major breakthroughs in the development, operation and mutual comparison of frequency standards based on the same hyperfine transition in caesium as used previously, but now incorporating the technique of laser cooling. Several cold-atom fountains have been developed. Mutual agreement to within about one part in 10(15) has been demonstrated for two of them operated side by side, and also for two operated simultaneously, in the US and Germany. This paper gives an overview of currently available commercial caesium clocks and primary standards developed in national metrology institutes.