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Abstract

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 $g_\text{th} =
2.000\,889\,894\,4\,(34)$ and experimental $g_\text{exp} =
2.000\,889\,888\,45\,(14)$ values of the $g$ factor of lithiumlike silicon
$^{28}$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.