First-principles study of transition metal dopants as spin qubits

Defect centers in solid-state materials have played an important role in the development of quantum information science as spin qubits. In recent years, many studies using optically detected magnetic resonance (ODMR) to manipulate the ground state spin of transition metal dopants have been reported. In this work, the tetrahedral ${d}^{2}$ and the octahedral ${d}^{3}$ transition metal ion systems are considered for the potential spin qubits, based on the ligand field theory and the basic requirements of general $\mathrm{\ensuremath{\Lambda}}$-type spin qubits. Then a computational scheme based on density functional theory is performed on the transition metal ligand structures $M{X}_{n}(M=\mathrm{V},\mathrm{Cr},\mathrm{Mn},\mathrm{Fe};X=\mathrm{N},\mathrm{O},\mathrm{C};\phantom{\rule{4pt}{0ex}}n=4, 6)$ in several typical hosts to analyze quantitatively their feasibility. The results on the charge transition levels, excitation, and excited-state structure relaxation energies of $M{X}_{n}$ show that some of them have the proper valence states to form the required ${d}^{2}$ or ${d}^{3}$ electronic systems and the appropriate excited-state level structure with the spin-selective transition. The results on the spontaneous emission lifetime show that the ${\mathrm{MnO}}_{4}$ structure is mostly suitable for spin polarization due to the fastest optical transition rates among these systems considered. Finally, some suggestions on determining potentially feasible systems activated by transition metal ions in solids are given.