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

Abstract

For decades, improved theory and experiment of the $n=2$ ${}^3$P fine structure of helium have allowed for increasingly-precise tests of quantum electrodynamics, determinations of the fine-structure constant $\alpha$, and limitations on possible beyond-the-Standard-Model physics. Here we use the new frequency-offset separated-oscillatory-fields (FOSOF) technique to measure the $2^3$P${}_2\to 2^3$P${}_1$ interval. Our result of $2291176590(25)$~Hz represents a major step forward in precision for helium fine-structure measurements.