Application of Atomic Hydrogen at Low Temperatures: The Recirculating Cryogenic Hydrogen Maser

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Abstract
Conventional hydrogen masers are the most stable of all atomic clocks with stabilities Δf/f of better than 10-15 for measuring times τ of 103 sec. A new generation of cryogenic hydrogen masers has the potential to improve this figure by 2 to 3 orders of magnitude. Here we describe how research on spin-polarized atomic hydrogen led to the construction at the University of British Columbia of a uniquely cryogenic hydrogen maser. In this maser, which operates at 0.5K and uses liquid helium coated walls throughout, H atoms circulate back and forth between a microwave pumped state selector and the maser cavity. The measurement of the frequency stability is presently limited by the frequency stability of the available references, but values of Δf/f-13 for τ=1 sec have been recently established. The successful development of rather different cryogenic masers at other institutions will also be discussed.

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