Martino TRASSINELLI

Quantum electrodynamics (QED), the quantum field theory that describes the interaction between light and matter is commonly regarded as the best tested quantum theory in modern physics. However, this claim is mostly based upon extremely precise studies performed in the domain of relatively low-field strengths and light atoms/ions. In the realm of very strong electromagnetic fields, as present e.g. in the heaviest highly-charged ions, QED calculations enter a qualitatively different non-perturbative regime (in Za with Z the nuclear charge and a the fine structure constant) with quantum vacuum fluctuations effects such as vacuum polarisation and self-energy strongly affecting the atomic binding energies. Experimental studies on this regime are very challenging due to the required extremely high ionization state of the investigated ions. Therefore, at present, precision of QED tests in the non-perturbative strong-field domain still remains considerably lower as compared to the perturbative regime. For these endeavours, precise understanding of the structure of highly-charged ions is of great importance to test bound-state QED in strongest Coulomb fields. We present here the most recent experimental developments for measuring of transition energy in heavy, few-electron atoms. In particular, we present a recent experiment on high-accuracy x-ray spectroscopy of two-, three-, and four-electron uranium ions (Z=92, i.e. Za=0.7). The obtained experimental accuracy allows to test, on the one hand, two-loop quantum electrodynamics effects for the first time in heavy helium-like atoms, and, on the other hand, to isolate the effects of electron-electron interaction in heavy bound systems. Such measurements rule out some of the most recent theoretical predictions.