Ultrahigh-Precision Measurement of the n=2 Triplet P Fine Structure of Atomic Helium Using Frequency-Offset Separated Oscillatory Fields

For decades, improved theory and experiment of the n = 2 P-3 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 technique to measure the 2(3)P(2) -> 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.