Ramsey Spectroscopy of the 2S Hyperfine Interval in Cryogenically-Cooled Hydrogen

Hydrogen, due to its simple structure, has been paramount to the development of quantum mechanics and quantum electrodynamics (QED). One of the most accurate physical measurements in hydrogen is the 1S hyperfine interval, HF_1S, with a fractional uncertainty of about 1 × 10^-12. However, the theoretical prediction for this interval is limited by complications arising from proton structure. The difference D₂₁ = HF_1S - 8HF_2S, where HF_2S is the hyperfine interval of the 2S state, nearly avoids proton structure uncertainty and is theoretically determined to fourth-order in bound-state QED. Currently, the experimental uncertainty for D₂₁ is about 20 times larger than the theory and is limited by the experimental determination of the 2S hyperfine interval. In this talk, we will discuss an ongoing measurement of the hydrogen 2S hyperfine interval using Ramsey spectroscopy on a cold (5 K) beam of atomic hydrogen.