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 \(2\,291\,176\,590(25)\)~Hz represents a major step forward in precision for helium fine-structure measurements.