  Single photon emitters (SPEs) in solids have emerged as promising candidates
for quantum photonic sensing, communications, and computing. Defects in
hexagonal boron nitride (hBN) exhibit high-brightness, room-temperature quantum
emission, but their large spectral variability and unknown local structure
significantly challenge their technological utility. Here, we directly
correlate hBN quantum emission with the material's local strain using a
combination of photoluminescence (PL), cathodoluminescence (CL) and nano-beam
electron diffraction. Across 40 emitters and 15 samples, we observe zero phonon
lines(ZPLs) in PL and CL ranging from 540-720 nm. CL mapping reveals that
multiple defects and distinct defect species located within an
optically-diffraction-limited region can each contribute to the observed PL
spectra. Local strain maps indicate that strain is not required to activate the
emitters and is not solely responsible for the observed ZPL spectral range.
Instead, four distinct defect classes are responsible for the observed emission
range. One defect class has ZPLs near 615 nm with predominantly matched CL-PL
responses; it is not a strain-tuned version of another defect class with ZPL
emission centered at 580 nm. A third defect class at 650 nm has low
visible-frequency CL emission; and a fourth defect species centered at 705 nm
has a small, ~10 nm shift between its CL and PL peaks. All studied defects are
stable upon both electron and optical irradiation. Our results provide an
important foundation for atomic-scale optical characterization of color
centers, as well as a foundation for engineering defects with precise emission
properties.
