Willis Lamb’s 1947 measurement of the tiny energy splitting between the 2S and 2P states of atomic hydrogen largely spawned the development of quantum electrodynamics (QED). Ever since, precision spectroscopy of simple atomic systems has been used to test the theory and refine our understanding of it. In the latest contribution to that effort,1 researchers led by Wim Vassen (Vrije University Amsterdam) have resolved—to a precision of 1.5 kHz, or 8 parts in 1012—an exceedingly weak IR transition between the triplet and singlet metastable states of atomic helium, 2 3S1 and 2 1S0. So highly forbidden is the excitation, which violates both spin and parity selection rules, that it has never before been seen, much less measured to high precision.

Individually, the states are energy levels of different species of helium—orthohelium, in which electron spins are parallel, and parahelium, in...

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