= Electron shelving
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Electron shelving distinguishes an internal <quantum state> outside a cycling optical transition from one that scatters many <photons>. In the mercury-ion example in <David J. Wineland>'s lecture, $S\leftrightarrow P$ excitation produces $194\ \mathrm{nm}$ <fluorescence>; transfer to a <metastable atomic level> $D$ suppresses it. A <photomultiplier> records bright/dark intervals. Coherent $282\ \mathrm{nm}$ interrogation precedes detection to avoid perturbation by the detection beam.<Nobel Lecture: Superposition, entanglement, and raising Schrödinger’s cat by David J. Wineland>[§III, pp. 1105–1106, Figs. 1–2: shelving, photomultiplier counts and alternating interrogation/detection]{ref}
“Dark” means weak scattering under a particular detection setting. The motional <ground state> is dark to an ideal red <motional sideband> for a different reason: there is no lower <number state>. It can still fluoresce strongly on a cycling detection transition. Shelving's population readout also need not preserve a <quantum superposition>'s <quantum coherence>; scattered <photons> carry information about the internal state, and their recoil can disturb motion.
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