Abstract
Hydrogen masers are presently the most stable atomic clocks for averaging times τ between 1 s and 1000 s. With the best conventional devices, frequency fluctuations of less than 10−15 for averaging times of about 1 hour are measured [1]. In 1978 it was realized by Crampton et al. [2] that cryogenic hydrogen masers (CHM) could have even better frequency stabilities, because of lower cavity thermal noise and the possibility of using cooled preamplifiers. In addition, it was noted that the spin exchange broadening at low temperatures is 1000 times smaller, which increases the optimal density by about the same factor and results in a much higher power output. Berlinsky and Hardy [3] concluded that the frequency stability of a cryogenic hydrogen maser, operated at 0.5 K, could be up to 1000 times better than the best state-of-the-art frequency sources, provided that the stability is still limited by thermal and receiver noise.
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