THlB-2 CESIUM BEAM FREQUENCY STANDARDSf

This historical review of the development of cesium beam frequency standards covers the period from the announcement of the first atomic frequency standard in 1949 to the present. It describes the concepts as well as the key factors affecting the development of the various cesium standards. HISTORY OF DEVELOPMENT While the world’s first atomic frequency standard, developed by Lyons and his colleagues [l] at the National Bureau of Standards (NBS), was completed in 1949, the basis for the development was established much earlier by the molecular beam work of the Nobel laureate, 1.1. Rabi. He used molecular beams to detect atomic and molecular resonances (transitions) with unprecedented accuracy. It wasn’t long before people realized that these resonances could also be used as frequency standards. in ammonia, performed no better than the best quartz oscillators of that period, but it opened the door to subsequent developments. At the same time that NBS was introducing its standard, Ramsey, another Nobel prize winner, was inventing the separated oscillatory field method [2]. In this method, a molecular beam passes through two regions of microwave field separated by some distance along the beam path. This was a major improvement over Rabi’s The first NBS standard, based on a transition earlier devices. It reduced the observed linewidth and eliminated the first-order Doppler shift. Following the lead of the molecular beam work in the United States, Essen and Parry [3] of the National Physical Laboratory in England adapted this method to cesium-beam frequency standards in the mid-1950’s . Cesium was favored for frequency standards for several reasons. Its relatively high resonance frequency near 9.2 GHz was a good match to the microwave technology that had been developed during World War 11. Also, this resonance was particularly narrow and relatively insensitive to magnetic fields. Cesium beams are easy to produce, and cesium is readily detected with a hot filament and current detector. Cesium was selected very early in the game; though much research was conducted using other atoms, none has yet emerged to replace cesium. The cesium devices were so successfbl that in 1967 the second was redefined as “the duration of 9 192 63 1 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.” This number was based on a comparison, over a 2 314 year period, of a cesium beam frequency standard and very carehl astronomical observations [4]. of the concepts developed in the 1950s and 1960s to a point where these standards could realize the atomic definition of the second with an uncertainty of less than 1 x Even during this period of gradual evolution, the uncertainties The next 25 years saw gradual improvement