Frequency standards based on atomic hydrogen

The current state of hydrogen maser frequency standards is briefly reviewed. Particular emphasis is placed on the discussion of physical mechanisms which affect long-term stability. While questions concerning absolute accuracy still remain, recent experiments suggest that long-term stability can be at least as good as the best primary standards (1 × 10^-13/year). This long-term stability can be realized by small passive hydrogen maser field standards, a fact which makes these attractive alternatives to cesium field standards. Hydrogen masers already exhibit the best short-term stability of any room-temperature atomic clock and this could be improved by a factor of 100 with the development of hydrogen masers operating at cryogenic temperatures.