  Color centers in hexagonal boron nitride (hBN) have recently emerged as
promising candidates for a new wave of quantum applications. Thanks to hBN's
high stability and 2-dimensional (2D) layered structure, color centers in hBN
can serve as robust quantum emitters that can be readily integrated into
nanophotonic and plasmonic structures on a chip. More importantly, the recently
discovered optically addressable spin defects in hBN provide a quantum
interface between photons and electron spins for quantum sensing applications.
The most well-studied hBN spin defects, the negatively charged boron vacancy
($V_B^-$) spin defects, have been used for quantum sensing of static magnetic
fields, magnetic noise, temperature, strain, nuclear spins, paramagnetic spins
in liquids, RF signals, and beyond. In particular, hBN nanosheets with spin
defects can form van der Waals (vdW) heterostructures with 2D magnetic or other
materials for in situ quantum sensing and imaging. This review summarizes the
rapidly evolving field of nanoscale and microscale quantum sensing with spin
defects in hBN. We introduce basic properties of hBN spin defects, quantum
sensing protocols, and recent experimental demonstrations of quantum sensing
and imaging with hBN spin defects. We also discuss methods to enhance their
sensitivity. Finally, we envision some potential developments and applications
of hBN spin defects.
