= Lamb shift
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A <quantum electrodynamics> correction separates <hydrogen atom> levels such as $2s_{1/2}$ and $2p_{1/2}$ that coincide in the simplest model.
The Lamb shift measures an <energy level> displacement beyond a specified <Dirac equation> reference. In the point-<proton>, infinite-nuclear-<mass> <Coulomb potential> model, $2S_{1/2}$ and $2P_{1/2}$ share <principal quantum number> $n=2$ and total electronic <angular momentum> $j=1/2$, hence <energy>, despite differing <orbital angular momentum> values.<Theory of the Lamb shift in hydrogen and light hydrogen-like ions by Vladimir Yerokhin and others>[Theory of the Lamb shift in hydrogen and light hydrogen-like ions, v3, §II, Eqs. (2)–(4)]{ref}
The <Lamb-Retherford experiment> converts a small <energy> difference into a large detector change: prepare a long-lived <quantum state>, transfer population into a short-lived one, and read the surviving beam.
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