First-principles predictions of qubits in defective MgS

In this paper, MgS is evaluated to be a suitable qubit host material due to its wide bandgap, being nuclear spin free, and its weak spin-orbit coupling. The point defects including vacancies, substitutions, and defect pairs were systematically investigated using first-principles calculations. Results show that the neutral ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}$ defect pair has ${C}_{3v}$ symmetry with the defect level sequence of ${E}_{1}>{E}_{exy}>{E}_{2}$. In addition, the net spin and defect levels of ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}$ can be tuned by charge states. The ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{2+}$ and ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{2\ensuremath{-}}$ defects are relatively stable with the charge transition level of \ensuremath{\sim}2.53 eV. The four charged defect states of ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{+}$, ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{2+}$, ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{\ensuremath{-}}$, and ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{2\ensuremath{-}}$ may be plausible qubits with nonzero spin, and their zero-phonon lines indicate the corresponding fluorescence wavelengths fall within the infrared band. Furthermore, the zero field splitting of the ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{+}$ and ${\mathrm{P}}_{\mathrm{S}}{\mathrm{V}}_{\mathrm{Mg}}^{\ensuremath{-}}$ shows that the values of parameter $D$ are in the range of microwaves. The hyperfine constants are roughly proportional to the spin difference charge density and exponentially decay as a function of the distance from the P site.