Robust microwave-optical photon conversion using cavity modes strongly hybridized with a color center ensemble

A microwave-optical converter with high efficiency (>50%) and low added noise (≪1 photon) could enable the creation of scalable optical quantum networks. However, integrated converters demonstrated to date are too lossy or weakly non-linear to provide this performance. Here we develop a theory of microwave-optical conversion employing an ensemble of spin-bearing color centers strongly coupled to a photonic resonator and/or a superconducting microwave resonator. We find a counterintuitive operating point where microwave and optical photons are tuned to bare center/cavity resonances, which, compared to the weak-coupling limit, has much stronger non-linearity, offering high efficiency with reduced pump- and center-induced losses, and that is robust to inhomogeneous broadening. Taking color center and optical pump-induced losses into account, we predict ~ 95% efficiency and added noise ≪ 1 quanta at low (μW) pump powers for both Er- and T-centers in Si. Our results open new pathways to build quantum networks.