Entanglement in Resonance Fluorescence

Resonance fluorescence, the interaction between an artificial atom and coherent light, has been the subject of fundamental research from the early days of quantum optics. In particular, the observation of antibunching from this interaction open the gate for fundamental research on sources of single photons. From a theoretical point of view, resonance fluorescence can be modelled as a two-level system (2LS) which is excited by a laser. When the intensity of such a laser is strong enough, the 2LS enters into the so-called Mollow regime, where the emission spectrum consists of a triplet of peaks, with the lateral peaks having half the intensity of the central one. The results from the previous paper showed that when the light emitted from the Mollow triplet is observed (by a detector or by another optical target, e.g., exciton-polaritons) the correlations between the emitted photons are spoilt. 

In my paper Entanglement in Resonance Fluorescence, I exploit the fact that observation modifies the photon correlations, and show that when the 2LS is driven by a laser off resonantly, the photons emitted from the lateral peaks of the Mollow triplet are i)heralded (the emission of a photon from the low energy peak announces the emission of a photons from the high energy peak) and ii) entangled in time and frequency. The latter is a consequence of the detection of the photons with a finite bandwidth; otherwise, the photons would be indistinguishable and the entanglement would vanish. Therefore, one can consider detuned resonance fluorescence as a source of entangled photon pairs. Notably, when the intensity of the laser that drives the 2LS is increase, the emission rate of the stream of entangled photons grows without being contaminated by higher-order processes, unlike what happens to sources based on, e.g., parametric down-conversion. Finally, we used this source of quantum light to drive exciton-polaritons and showed that, although they are embedded in a highly-dissipative medium, their steady-state is given by a Bell state, namely, a maximally entangled state.