  We investigate theoretically the spin purity of single holes confined in
vertically coupled GaAs/AlGaAs quantum dots (QDs) under longitudinal magnetic
fields. A unique behavior is observed for triangular QDs, by which the spin is
largely pure when the hole is in one of the dots, but it becomes strongly mixed
when an electric field is used to drive it into molecular resonance. The spin
admixture is due to the valence band spin-orbit interaction, which is greatly
enhanced in C3h symmetry environments. The strong yet reversible electrical
control of hole spin suggests that molecules with C3-symmetry QDs, like those
obtained with [111] growth, can outperform the usual C2-symmetry QDs obtained
with [001] growth for the development of scalable qubit architectures.
