  Solid-state single-photon emitters (SPEs) such as the bright, stable,
room-temperature defects within hexagonal boron nitride (hBN) are of increasing
interest for quantum information science applications. To date, the atomic and
electronic origins of SPEs within hBN are not well understood, and no studies
have reported photochromism or explored cross-correlations between hBN SPEs.
Here, we combine irradiation-time dependent measures of quantum efficiency and
microphotoluminescence (${\mu}$PL) spectroscopy with two-color Hanbury
Brown-Twiss interferometry to enable an investigation of the electronic
structure of hBN defects. We identify photochromism in a hBN SPE that exhibits
cross-correlations and correlated quantum efficiencies between the emission of
its two zero-phonon lines.
