Increasing accuracy of the theory and experiment of the n=23P fine
structure of helium has allowed for increasingly-precise tests of quantum
electrodynamics (QED), determinations of the fine-structure constant α,
and limitations on possible beyond-the-Standard-Model physics. Here we present
a 2~part-per-billion (ppb) measurement of the J=1-to-J=0 interval.
The measurement is performed using frequency-offset
separated-oscillatory-fields. Our result of 29616955018(60)~Hz
represents a landmark for helium fine-structure measurements, and, for the
first time, will allow for a 1-ppb determination of the fine-structure constant
when QED theory for the interval is improved.Comment: 6 pages, 6 figures. arXiv admin note: substantial text overlap with
arXiv:1807.0792