Optically addressable spin defects in boron nitride nanotubes (BNNTs) are reported, exhibiting a spin S = 1/2 ground state without an intrinsic quantization axis, enabling orientation-independent magnetic field sensing. This feature allows for anisotropic magnetization observation in 2D magnets and the development of scanning probe magnetometry using a BNNT on an AFM cantilever. The findings suggest potential for atomic-scale quantum sensing of magnetic fields in any direction.
