Testing interelectronic interaction in lithium-like tin

Magnetic moments of bound-electron systems are a sensitive tool for testing fundamental interactions. The <i>g</i> factors of lithium-like ions have been rigorously studied in recent years, enabling insights into the relativistic interelectronic effects. In this work, we present the <i>g</i>-factor measurement of lithium-like tin, accurate to 0.5 parts per billion, as well as ab initio theoretical calculations that include an advanced treatment of the interelectronic interaction. We further improved the prediction by using the experimental result for the hydrogen-like tin <i>g</i> factor, inferring from it the unknown higher-order quantum electrodynamic (QED) effects. The observed agreement independently confirms the revised theory at a previously inaccessible high atomic number <i>Z</i> of 50, where QED effects are considerably larger.