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With hydrogen maser clocks now routinely performing near the theoretical limit set by thermal noise, the devices have become the most accurate frequency standard available to science. At room temperature this performance corresponds to a stability of 6 parts in 1016 measured over one-hour averaging intervals. Such masers are currently being developed and built at the Harvard-Smithsonian Center for Astrophysics for very long base line interferometry and satellite-tracking applications.

The prospects for further stability have been enhanced as a result of recent experimental tests with a cryogenic atomic hydrogen maser. Successful oscillation at temperatures down to 25°K was attained recently. This was made possible by the introduction of carbon tetrafluoride into the supercooled maser cavity, where it froze on the cavity walls to form a surface similar to that of the Teflon walls used in room temperature masers. This coating successfully preserved the phase of the oscillating hydrogen atoms as they collided with the cavity walls. At a temperature of 300°K the available stability of oscillation was calculated to be below 1 part in 1016, a factor of 6 better than the hydrogen masers currently in use.