Overview
- A Max Planck–led team measured the hydrogen 2S–6P transition using Doppler‑free single‑photon spectroscopy on a 4.6 K atomic beam.
- The extracted proton charge radius of 0.8406 femtometres matches muonic‑hydrogen determinations and agrees with the updated CODATA reference.
- Experimental and theoretical values concur at about 0.7 parts per trillion, establishing a new benchmark for bound‑state QED tests.
- The determination is 2.5 times more precise than the next‑best atomic‑hydrogen result and six times better than the team’s earlier 2S–4P study.
- The authors report the previous electron–muon discrepancy is excluded at more than five standard deviations and say the methods can extend to other transitions and to deuterium.