We report a new test of quantum electrodynamics (QED) for the w (1 s 2 p P 1 1→ 1 s 2 S 0
1) x-ray resonance line transition energy in heliumlike titanium. This measurement is one of
few sensitive to two-electron QED contributions. Systematic errors such as Doppler shifts are
minimized in our experiment by trapping and stripping Ti atoms in an electron beam ion trap
and by applying absolute wavelength standards to calibrate the dispersion function of a
curved-crystal spectrometer. We also report a more general systematic discrepancy between …

We report the measurement of the w (1s2p 1P1! 1s2 1S0) resonance line transition energy
in helium-like titanium. Our result, 4749.85 (7) eV, deviates from the most recent ab initio
prediction by three times our experimental uncertainty and by many times more than the
previously estimated uncertainty in the prediction. This measurement is one of few sensitive
to two-electron quantum electrodynamics (QED) contributions. Systematic errors such as
Doppler shifts are minimised in our experiment by trapping and stripping Ti atoms in an …