A hydrogen maser uses a ground-state hyperfine structure transition of the hydrogen atom to provide a stable microwave signal. A beam source and magnetic state selector prepare a favorable atomic population; a storage bulb within a microwave resonator lets atoms interact with the field for an extended time.
An active device sustains oscillation through stimulated emission, while a passive arrangement interrogates the atomic response with an external signal. Long interrogation supports a narrow frequency reference, but magnetic shifts, wall interactions and cavity resonance still affect its measured output. Short-term stability and long-term accuracy are different properties. [ref]
https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-h. NIST Time and Frequency from A to Z, Hydrogen maser: atomic transition, stability and cavity-frequency drift
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