= Hydrogen maser
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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. 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]{ref}
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