g Factor of Boronlike Argon ^{40}Ar^{13+}.

We have measured the ground-state g factor of boronlike argon ^{40}Ar^{13+} with a fractional uncertainty of 1.4×10^{-9} with a single ion in the newly developed Alphatrap double Penning-trap setup. The value of g=0.663 648 455 32(93) obtained here is in agreement with our theoretical prediction of 0.663 648 12(58). The latter is obtained accounting for quantum electrodynamics, electron correlation, and nuclear effects within the state-of-the-art theoretical methods. Our experimental result distinguishes between existing predictions that are in disagreement, and lays the foundations for an independent determination of the fine-structure constant.