Separated oscillatory-field microwave measurement of the2P31–2P32fine-structure interval of atomic helium

The $2{{}^{3}P}_1–2{{}^{3}P}_2$ interval in helium is measured using microwave separated oscillatory fields. Our measured result is $2291177.53\pm 0.35\text{kHz}$ . A disagreement with theory of over 15 kHz indicates a major problem with two-electron QED calculations. If this disagreement is resolved, measurements of both $2{}^{3}P$ fine-structure intervals at this accuracy would lead to a six-parts-per-billion determination of the fine-structure constant.