Eric A Hessels & Matthew C. GeorgeHeydarizadmotlagh, Farshad2025-07-232025-07-232025-05-202025-07-23https://hdl.handle.net/10315/43055Increasing accuracy of the theory and experiment of the 2^3 P 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. The work presented is a 2-part-per-billion (ppb) measurement of the 𝐽=1-to-𝐽=0 interval. The measurement is performed using the frequency-offset separated-oscillatory-fields (FOSOF) method. The 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.Author owns copyright, except where explicitly noted. Please contact the author directly with licensing requests.PhysicsAtomic physicsQuantum physicsElectronic Thesis or Dissertation2025-07-23PhysicsQuantum physicsHeliumFine structureFine-structure constantAtomic physicsPrecision measurementSpectroscopyMicrowavesSeparated oscillatory fieldsFOSOF