Precision measurement of the n=2 triplet P J=1-to-J=0 fine structure of atomic helium using frequency-offset separated oscillatory fields

Increasing accuracy of the theory and experiment of the n=2 ³P 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. A helium beam is produced using a liquid-nitrogen-cooled dc-discharge source, and is intensified using a two-dimentional magnetooptical trap. The microwave measurement is performed using frequency-offset separated oscillatory fields (FOSOF) [1]. Laser excitation to a Rydberg state, followed by Stark ionization allows for efficient detection. Our result of 29,616,955,018(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.

[1] A. C. Vutha and E. A. Hessels, Phys. Rev. A 92, 052504 (2015).