  Quantum microscopes based on solid-state spin quantum sensors have recently
emerged as powerful tools for probing material properties and physical
processes in regimes not accessible to classical sensors, especially on the
nanoscale. Such microscopes have already found utility in a variety of
problems, from imaging magnetism and charge transport in nanoscale devices, to
mapping remanent magnetic fields from ancient rocks and biological organisms.
However, applications of quantum microscopes have so far relied on sensors
hosted in a rigid, three-dimensional crystal, typically diamond, which limits
their ability to closely interact with the sample under study. Here we
demonstrate a versatile and robust quantum microscope using quantum sensors
embedded within a thin layer of a van der Waals (vdW) material, hexagonal boron
nitride (hBN). To showcase the capabilities of this platform, we assemble
several active vdW heterostructures, with an hBN layer acting as the quantum
sensor. We demonstrate time-resolved, simultaneous temperature and magnetic
imaging near the Curie temperature of a vdW ferromagnet as well as apply this
unique microscope to map out charge currents and Joule heating in graphene. By
enabling intimate proximity between sensor and sample, potentially down to a
single atomic layer, the hBN quantum sensor represents a paradigm shift for
nanoscale quantum sensing and microscopy. Moreover, given the ubiquitous use of
hBN in modern materials and condensed matter physics research, we expect our
technique to find rapid and broad adoption in these fields, further motivated
by the prospect of performing in-situ chemical analysis and noise spectroscopy
using advanced quantum sensing protocols.
