A carbon dimer defect as a spin qubit candidate in hexagonal boron nitride: an <i>ab-initio</i> study

Hexagonal boron nitride has been recently found to host a variety of quantum defects that are potentially useful for advancing solid-state quantum technologies. However, optically addressable spin quantum bits (qubits), such as the diamond NV center, have not yet been reported in h-BN. In this talk, we propose a carbon dimer(C₂) defect as a promising candidate of optically active spin qubit in h-BN. We use first-principles density functional theory to investigate its structural and electronic properties. We show that the C₂ defect features a non-zero spin ground state (S=1), arising from defect-induced deep levels in the band gap of the host material, and localized unpaired electrons. We calculate the defect formation energy of the C₂ defect and other C-related defects in h-BN and we show that the C₂ defect is energetically stable. To assist potential future experiments, we report the zero-phonon line, the zero-field splitting tensor, and the hyperfine tensor of the C₂ defect at various DFT levels of theory, including the HSE hybrid functional. Our study aimed at searching coherent spin qubits in h-BN can be potentially applied to a wide variety of other two-dimensional van der Waals materials systems.