# Defects as entangled photon pair sources

Nonclassical states of light are important resources for quantum technologies, such as quantum information processing, networking, and metrology. Entangled photon pairs, in particular, have applications in solid-state quantum repeaters, a crucial component of long-distance quantum networking that overcomes transmission loss by leveraging the effects of entanglement swapping and quantum teleportation. Despite the diverse applications for such nonclassical states of light, methods for generating them deterministically remain limited. In this paper, we developed the theoretical basis for a deterministic entangled photon pair source from a pair of dipole-coupled, three-level, and generic quantum emitters. One further question spawning from this study is, Can this model setting tight constraints on the properties of the quantum emitters be generalized? Specifically, what happens if, for instance, the two excited state transitions are not perfectly orthogonally polarized (as is likely the case in defects whose geometries do not have square symmetries), any of the electronic states have permanent dipole moments, the transitions are circularly instead of linearly polarized, losses and decoherence are non-trivial, or there are additional excited state transitions beyond the 2 necessary for the proposal? (To understand the importance of these questions in context, one should read the original paper.) Based on these better understood constraints, we can then search for the right defect complex to experimentally realize this proposal, probably by first using group symmetry-based arguments to narrow our search and then plumbing experimental literature or running first-principles calculations to predict at what photon frequencies we can expect emission and at which efficiencies. The results of this project overall will drastically narrow the search for scientists who design quantum devices based on defect centers to realize on-chip, solid-state, and determission emission of entangled photon pairs.