Established Microwave Frequency Standards
Atomic and molecular beam physics, which began with the experiment of 0. Stern and W. Gerlach in 1921, is the foundation of today’s atomic time and frequency standards. The following papers discuss progress in the area of microwave frequency standards and clocks. Microwave devices are defined, for our purposes here, as those atomic resonators which feature resonances in the microwave region of the electromagnetic spectrum. The combination of atomic and molecular beam physics with the idea to use an oscillator-driven magnetic field to induce transitions was born in the 1930’s and led to molecular beam spectroscopy. The invention of efficient sources for microwave radiation in the 1940’s as a result of the Second World War made practical devices possible. The manipulation of atomic excitations through optical pumping together with the invention of the maser-laser principle in the 1950’s have combined to allow novel realizations of atomic resonators. The advent of solid state electronics permitted the construction of clocks and frequency standards of considerable reliability, together with highly accurate performance.