Hg{sup +} frequency standards

In this paper we review the development of Hg{sup +} microwave frequency standards for use in high reliability and continuous operation applications. In recent work we have demonstrated short-term frequency stability of 3x10{sup -14}/{radical}{tau} when a cryogenic oscillator of stability 2-3x10{sup -15} was used as the local oscillator. The trapped ion frequency standard employs a {sup 202}Hg discharge lamp to optically pump the trapped {sup 199}Hg{sup +} clock ions and a helium buffer gas to cool the ions to near room temperature. We describe a small Hg{sup +} ion trap based frequency standard with an extended linear ion trap (LITE) architecture which separates the optical state selection region from the clock resonance region. This separation allows the use of novel trap configurations in the resonance region since no optical pumping is carried out there. A method for measuring the size of an ion cloud inside a linear trap with a 12-rod trap is currently being investigated. At {approx}10{sup -12}, the 2{sup nd} order Doppler shift for trapped mercury ion frequency standards is one of the largest frequency offsets and its measurement to the 1% level would represent an advance in insuring the very long-term stability of these standards to the 10{sup -14} or better level. Finally, we describe atomic clock comparison experiments that can probe for a time variation of the fine structure constant, {alpha}=e{sup 2}/2{pi}hc, at the level of 10{sup -20}/year as predicted in some Grand Unified String Theories.