Phonon-induced multicolor correlations in hBN single-photon emitters

Color centers in hexagonal boron nitride have shown enormous promise as single-photon sources, but a clear understanding of the electron-phonon interaction dynamics is critical to developing the material for quantum communications or quantum simulations. We demonstrate photon antibunching in the filtered auto- and cross-correlations gₗₘ⁽²⁾(τ) between zero-, one-, and two-phonon replicas of defect luminescence. With no background correction, we observe a single-photon purity of g⁽²⁾(0)=0.20 in a phonon replica and cross-spectral correlations of gₗₘ⁽²⁾(0)=0.18 between a phonon replica and the zero-phonon line. Moreover, we combine a violation of the Cauchy-Schwarz inequality and a phenomenological model of the phonon-mediated emission cross section to distinguish a low quantum-efficiency defect from phonon replicas of a bright defect.