Midgap state requirements for optically active quantum defects

Point defects in semiconducting hosts are promising for quantum information science (QIS). For instance, they can be used as spin-photon interfaces and a handful "quantum defects" are currently known and studied (e.g., NV center in diamond, T center in silicon, ...). A common criteria for the viability of such a "quantum defect" especially when studied through first principles computations is to require single particle defect states far from the band edges that can be excited by photons. Based on an analysis of currently studied quantum defects in silicon and diamond, we present evidences that such a model of two defect states far from the band edge might not be necessary. We argue that alternative electronic structures are viable as well and discuss their pros and cons. Our study refines the requirement necessary for viable quantum defects and will help the identification of new color centers for QIS using for instance first principles computations.