Ultra-stable Hg+

Abstract
We report the development of a fieldable frequency standard based on 199Hg+ ions confined in a hybrid r.f./dc linear ion trap. This trap permits storage of large numbers of ions with reduced susceptibility to the second-order Doppler effect caused by the r.f. confining fields. A 160 mHz wide atomic resonance line for the 40·5 GHz clock transition is used to steer the output of a 5 MHz crystal oscillator to obtain a stability of 2 × 10−15 for 24 000 s averaging times. For longer averaging intervals, measurements are limited by instabilities in available hydrogen maser frequency standards. Measurements with 37 mHz linewidth for the Hg+ clock transition demonstrate that the inherent stability for this frequency standard is at least as good as 1 × 10−15.

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