A survey of atomic frequency standards

The survey reviews the historical background and development of atomic frequency standards leading to the most recent achievements. A qualitative discussion of underlying principles is also included. Modern atomic frequency standards produced in the United States are illustrated, and performance data reflecting the present state of the art is presented. The comparison of the different types of atomic frequency standards is complicated by the many quantitative factors relating to performance and physical characteristics which differ in each case. However, on a qualitative basis it may be stated that rubidium gas cell frequency standards are, at the present time, the most compact, light weight atomic oscillators, and they have a high level of short-term stability. The long-term stability in this case exceeds that of the best quartz crystal oscillators by two or more orders of magnitude without the need for long warm-up or stabilization periods. Calibration in terms of a primary standard is required. The cost of rubidium frequency standards is less than that of other atomic standards. Cesium atomic beam frequency standards are somewhat larger and heavier than rubidium standards, and the short-term stability is limited by shot noise in the cesium tube resonance signal. However, the long term stability is very high, and the limits have not yet been measured for very long averaging periods. The cesium standard also has a high degree of intrinsic reproducibility which qualifies it for service as a primary standard without the need for calibration. The atomic hydrogen maser, at present, has the highest degree of short-term stability, long-term stability, and intrinsic reproducibility when compared with the other commercially produced atomic frequency standards. It is also the largest and the cost is influenced by the more advanced performance characteristics. New developments which are in progress include the thallium atomic beam controlled frequency standard and the active rubidium maser. Along with these advances, new improvements will continue to be made in the various characteristics of the established types of atomic frequency standards. It is likely that their relative qualities will remain approximately the same, as the absolute values of the characteristics a- re advanced.