The Hydrogen Maser

We come now to consider what proved to be the culmination of efforts to enhance the spectral resolution of atomic resonance machines: the hydrogen maser, one of the most stable of all present-day atomic frequency standards. Few other microwave quantum devices exceed its overall mid-term frequency stability. Conceptually, the H-maser was a natural outgrowth of the continuing experimental drive to improve the spectral resolution of atomic beam resonance machines by increasing the interaction time between the atoms and the resonant field. Since this long predates the development of techniques for cooling atoms with laser radiation, this was to be achieved by confining the atoms interacting with the field within a space defined by inert walls; however, few would have predicted the degree of inertness exhibited by one fluorocarbon polymer named Teflon and the extraordinary length of perturbation-free interaction time it made possible.