Symmetric carbon tetramers forming chemically stable spin qubits in hexagonal boron nitride*

Point defect quantum bits in semiconductors have the potential to revolutionize sensing at atomic scales. Currently, vacancy related defects, such as the NV center in diamond and the VB− in hexagonal boron nitride (hBN), are at the forefront of high spatial resolution and low dimensional sensing. On the other hand, vacancies' reactive nature and instability at the surface limit further developments. Here, we present the symmetric carbon tetramers (C4) in hBN and propose them as a chemically stable spin qubit for sensing in low dimensions. We utilize periodic-DFT and many-body quantum chemistry approaches to reliably and accurately predict the electronic, optical, and spin properties of the studied defect, such as radiative and nonradiative lifetimes and photoluminescence spectra. We show that the nitrogen centered symmetric carbon tetramer gives rise to spin state dependent optical signals with strain sensitive intersystem crossing rates.