  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 study the symmetric carbon
tetramers in hBN and propose them as a chemically stable spin qubit for sensing
in low dimensions. We utilize periodic-DFT and quantum chemistry approaches to
reliably and accurately predict the electronic, optical, and spin properties of
the studied defect. We show that the nitrogen centered symmetric carbon
tetramer gives rise to spin state dependent optical signals with strain
sensitive intersystem crossing rates. Furthermore, the weak hyperfine coupling
of the defect to their spin environments results in a reduced electron spin
resonance linewidth that may enhance sensitivity.
