Generation of polarization-entangled photon pairs from two interacting quantum emitters

Entangled photon pairs are key elements in quantum communication and quantum cryptography. State-of-the-art sources of entangled photons are mainly based on parametric-down conversion from nonlinear crystals, which is probabilistic in nature, and on cascade emission from biexciton quantum dots, which finds difficulties in generating entangled photons in the visible regime. Here, we provide a proof-of-principle demonstration that polarization-entangled photon pairs can be emitted from two interacting quantum emitters with two-level-system behaviour and perpendicular transition dipole moments. These emitters can represent a large variety of systems (e.g., organic molecules, quantum dots and diamond-color centers) offering a large technological versatility, for example in the spectral regime of the emission. We show that a highly entangled photon pair can be post-selected from this system by including optical filters. Additionally, we verify that the photon entanglement is not significantly affected by small changes in the detection directions and in the orientation between the dipole moments.