Symmetry-Protected Two-Level System in the H₃ Center Enabled by a Spin–Photon Interface: A Competitive Qubit Candidate for the NISQ Technology

Recent technological advances in the noisy intermediate-scale quantum (NISQ) era are promising. Optically addressable color centers in semiconductors can achieve spin localization and are the most promising spin qubit candidates for NISQ technologies. Exploring a suitable and scalable atomic-like color center is a prerequisite in this context. Here, a symmetry-protected two-level system (qubit) derived from a set of naturally separated Bell states in the H₃ center with C_2v symmetry is characterized. The characterized qubit significantly reduces magnetic noise compared with a degenerate triplet in C_3v systems for conventional qubits. The Hamiltonian, including the Coulomb interaction, spin–orbit coupling, and spin–spin interaction, is comprehensively developed using a combination of first-principle calculations and group theory analyses. Consequently, an intrinsic spin–phonon interface embedded in an optical spin-polarization loop can enable an effective interrogation of the information of the two-level system. This study not only paves the way for developing further quantum information science applications utilizing the H₃ center but also provides a competitive qubit candidate for the NISQ era.