  Electric quadrupole (E2) matrix elements provide a measure of nuclear
deformation and related collective structure. Ground-state quadrupole moments
in particular are known to high precision in many p-shell nuclei. While the
experimental electric quadrupole moment only measures the proton distribution,
both proton and neutron quadrupole moments are needed to probe proton-neutron
asymmetry in the nuclear deformation. We seek insight into the relation between
these moments through the ab initio no-core configuration interaction (NCCI),
or no-core shell model (NCSM), approach. Converged ab initio calculations for
quadrupole moments are particularly challenging, due to sensitivity to
long-range behavior of the wave functions. We therefore study more
robustly-converged ratios of quadrupole moments: across mirror nuclides, or of
proton and neutron quadrupole moments within the same nuclide. In calculations
for mirror pairs in the p shell, we explore how well the predictions for mirror
quadrupole moments agree with experiment and how well isospin (mirror) symmetry
holds for quadrupole moments across a mirror pair.
