  Single photon emitters in 2D hexagonal boron nitride (hBN) have attracted a
considerable attention because of their highly intense, stable, and
strain-tunable emission. However, the precise source of this emission, in
particular the detailed atomistic structure of the involved crystal defect,
remains unknown. In this work, we present first-principles calculations of the
vibrationally resolved optical fingerprint of the spin-triplet (2)(_^3)B_1 to
(1)(_^3)B_1 transition of the VNCB point defect in hBN. Based on the excellent
agreement with experiments for key spectroscopic quantities such as the
emission frequency and polarization, the photoluminescence (PL) line shape,
Huang-Rhys factor, Debye-Waller factor, and re-organization energy, we
conclusively assign the observed single photon emission at ~2eV to the VNCB
defect. Our work thereby resolves a long-standing debate about the exact
chemical nature of the source of single photon emission from hBN and
establishes the microscopic understanding necessary for controlling and
applying such photons for quantum technological applications.
