Entanglement in Resonance Fluorescence

Particle entanglement is a fundamental resource upon which are based many quantum technologies. In this Article, we introduce a new source of entangled photons based on Resonance Fluorescence delivering photon pairs as a superposition of vacuum and the Bell state $$\left\vert {{{\Phi }}}^{-}\right\rangle$$ . Our proposal relies on the emission from the satellite peaks of a two-level system driven by a strong off-resonant laser, whose intensity controls the frequencies of the entangled photons. Notably, such a frequency tuning can be done without decreasing the degree of entanglement between the photons and, unlike current technologies, the intensity of our source can be increased without the risk of spoiling the signal by including higher-order processes into the emission. Finally, we illustrate the power of our novel source by exciting an ubiquitous condensed-matter system, namely exciton-polaritons, and show that they are left in a maximally entangled steady state.