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Increasing accuracy of the theory and experiment of the $n$$=$$2$ $^3$P finestructure of helium has allowed for increasingly-precise tests of quantumelectrodynamics (QED), determinations of the fine-structure constant $\alpha$,and limitations on possible beyond-the-Standard-Model physics. Here we presenta 2~part-per-billion (ppb) measurement of the $J$$=$$1$-to-$J$$=$$0$ interval.The measurement is performed using frequency-offsetseparated-oscillatory-fields. Our result of $29\,616\,955\,018(60)$~Hzrepresents a landmark for helium fine-structure measurements, and, for thefirst time, will allow for a 1-ppb determination of the fine-structure constantwhen QED theory for the interval is improved.

Precision measurement of the n=2 triplet P J=1-to-J=0 fine structure of atomic helium using frequency-offset separated oscillatory fields