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For decades, improved theory and experiment of the $n=2$ $^3$P fine structureof helium have allowed for increasingly-precise tests of quantumelectrodynamics, determinations of the fine-structure constant $\alpha$, andlimitations on possible beyond-the-Standard-Model physics. Here we use the newfrequency-offset separated-oscillatory-fields (FOSOF) technique to measure the$2^3$P$_2\!\!\to\!2^3$P$_1$ interval. Our result of $2\,291\,176\,590(25)$~Hzrepresents a major step forward in precision for helium fine-structuremeasurements.

Ultrahigh-precision measurement of the $n=2$ triplet P fine structure of atomic helium using frequency-offset separated oscillatory fields