Phonon-assisted luminescence in defect centers from many-body perturbation theory

Phonon-assisted luminescence is a key property of defect centers insemiconductors, and can be measured to perform the readout of the informationstored in a quantum bit, or to detect temperature variations. The investigationof phonon-assisted luminescence usually employs phenomenological models, suchas that of Huang and Rhys, with restrictive assumptions that can fail to bepredictive. In this work, we predict luminescence and study exciton-phononcouplings within a rigorous many-body perturbation theory framework, ananalysis that has never been performed for defect centers. In particular, westudy the optical emission of the negatively-charged boron vacancy in 2Dhexagonal boron nitride, which currently stands out among defect centers in 2Dmaterials thanks to its promise for applications in quantum information andquantum sensing. We show that phonons are responsible for the observedluminescence, which otherwise would be dark due to symmetry. We also show thatthe symmetry breaking induced by the static Jahn-Teller effect is not able todescribe the presence of the experimentally observed peak at 1.5 eV.