ECAMP 15

Transition energy measurements in heavy, few-electron ions are unique tools to test bound-state quantum electrodynamics (QED) in extremely high Coulomb fields, where perturbative methods cannot be implemented. In such fields, the effects of the quantum vacuum fluctuations on the atomic energies are enhanced by several orders of magnitude with respect to light atoms. However, up to now, experiments have been unable to achieve sensitivity to higher-order (two-loop) QED effects in this strong regime. Here we present a novel multi-reference method based on Doppler-tuned x-ray emission from fast uranium ions stored in the ESR ring of the GSI/FAIR facility. By accurately measuring the relative energies between $2p_{3/2} \to 2s_{1/2}$ transitions in two-, three-, and four-electron uranium ions, we were able, for the first time in this regime, to disentangle and test separately high-order (two-loop) one-electron and two-electron QED effects, and set a new important benchmark for this theory in the strong field domain [1]. The achieved accuracy of 37 parts per million allows us to discriminate between different theoretical approaches developed throughout the last decades for describing He-like systems. Experimental outlooks will be presented, as new calibration schemes and the implementation of a new time- and position-sensitive detector, based on Timepix3 technology. These considered improvements will allow for a reduction of the uncertainties by another order of magnitude, lower than the present theoretical prediction uncertainties and nuclear deformation effects.

[1] R. Loetzsch et al., Nature 625, 673-678 (2024).