  Optically-active spin defects hosted in hexagonal boron nitride (hBN) are
promising candidates for the development of a two-dimensional (2D) quantum
sensing unit. Here, we demonstrate quantitative magnetic imaging with hBN
flakes doped with negatively-charged boron-vacancy (V$_{\rm B}^-$) centers
through neutron irradiation. As a proof-of-concept, we image the magnetic field
produced by CrTe$_2$, a van der Waals ferromagnet with a Curie temperature
slightly above $300$ K. Compared to other quantum sensors embedded in 3D
materials, the advantages of the hBN-based magnetic sensor described in this
work are its ease of use, high flexibility and, more importantly, its ability
to be placed in close proximity to a target sample. Such a sensing unit will
likely find numerous applications in 2D materials research by offering a simple
way to probe the physics of van der Waals heterostructures.
