The recently established agreement between experiment and theory for the $g$ factors of lithiumlike silicon and calcium ions manifests the most stringent test of the many-electron bound-state quantum electrodynamics (QED) effects in the presence of a magnetic field. In this Letter, we present a significant simultaneous improvement of both theoretical $g_{\mathrm{th}}=2.0008898944(34)$ and experimental $g_{\exp }=2.00088988845(14)$ values of the $g$ factor of lithiumlike silicon ${{}^{28}\mathrm{Si}}^{11+}$. The theoretical precision now is limited by the many-electron two-loop contributions of the bound-state QED. The experimental value is accurate enough to test these contributions on a few percent level.

• Received 27 March 2019
• Revised 31 August 2019

DOI:https://doi.org/10.1103/PhysRevLett.123.173001