  Highly charged ions (HCIs) have drawn significant interest in quantum
metrology and in search for new physics. Among these, Ni12+ is considered as
one of the most promising candidates for the next generation of HCI optical
clocks, due to its two E1-forbidden transitions M1 and E2, which occur in the
visible spectral range. In this work, we used the Shanghai-Wuhan Electron Beam
Ion Trap to perform a high-precision measurement of the M1 transition
wavelength. Our approach involved an improved calibration scheme for the
spectra, utilizing auxiliary Ar+ lines for calibration and correction. Our
final measured result of the M1 transition wavelength demonstrates a five-fold
improvement in accuracy compared to our previous findings, reaching the
sub-picometer level accuracy. In combination with our rigorous atomic-structure
calculations to capture the electron correlations and relativistic effects, the
quantum electrodynamic (QED) corrections were extracted. Moreover, comparing
with an estimate of the one-electron QED contributions by using the GRASP2018
package, we found that the present experimental accuracy is high enough for
testing the higher-order QED corrections for such a complex system with four
electrons in the p subshell.
