Theory of optical spin-polarization of axial divacancy and nitrogen-vacancy defects in 4H-SiC

The neutral divacancy and the negatively charged nitrogen-vacancy defects in 4H-silicon carbide (SiC) are two of the most prominent candidates for functioning as room-temperature quantum bits (qubits) with telecommunication-wavelength emission. Nonetheless, the pivotal role of electron-phonon coupling in the spin-polarization loop is still unrevealed. In this work, we theoretically investigate the microscopic magneto-optical properties and spin-dependent optical loops utilizing the first-principles calculations. First, we quantitatively demonstrate the electronic level structure, assisted by symmetry analysis. Moreover, the fine interactions, including spin-orbit coupling and spin-spin interaction, are fully characterized to provide versatile qubit functional parameters. Subsequently, we explore the electron-phonon coupling, encompassing dynamics- and pseudo-Jahn-Teller effects in the intersystem crossing transition. In addition, we analyze the photoluminescence lifetime based on the major transition rates in the optical spin-polarization loop. We compare two promising qubits with similar electronic properties, but their respective rates differ substantially. Finally, we detail the threshold of the optically detected magnetic resonance (ODMR) contrast for further optimization of the qubit operation. This work not only reveals the mechanism underlying the optical spin-polarization but also proposes productive avenues for optimizing quantum information processing tasks based on the ODMR protocol.