= Cesium-beam clock procedure
An oven supplies a collimated <caesium> beam. State-selecting magnets precede and follow two coherent <microwave> interaction regions; a weak static <magnetic field> separates sublevels. The clock transition connects $F=3,m_F=0$ and $F=4,m_F=0$. Transferred atoms reach a heated ionizing collector. A detector scan identifies the resonance before locking.<The Physical Basis of Atomic Frequency Standards by A. S. Risley>[April 1971 edition, §5.3, pp. 36–38, Fig. 7]{ref}
\Image[/cesium-beam-clock-feedback.png]
{title=A beam clock compares atoms with an oscillator and feeds back the error}
{description=Original functional diagram of the <cesium-beam clock procedure>; magnetic selection, schematic paths.}
Here $F$ labels the total <angular momentum> and $m_F$ its projection. The selected pair is a <two-level system> inside a larger set of atomic <energy levels>; treating every oven atom as a prepared <qubit> would overstate the preparation. A measured detector signal may be modeled as $S(\nu)=S_{\mathrm{bg}}+A\overline P(2\pi(\nu_0-\nu))$, with background $S_{\mathrm{bg}}$ and gain $A>0$. Background subtraction and stable transmission are necessary before interpreting the signal as a transition <probability>. The <velocity-averaged Ramsey line> provides one explicit ideal example; its chosen dimensions are not historical specifications.
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