Hybrid quantum frequency conversion

In this work, we introduce the concept of hybrid quantum frequency conversion (HQFC), the transduction of quantum information between different degrees of freedom of light fields with distinct colors. We develop the theory of HQFC and devise an experiment to transfer a quantum state from the polarization of a photon at Telecom wavelengths to the spatial mode of a visible photon. We benchmark our experiment by performing HQFC in one out of a pair of photons sharing a polarization-entangled quantum state, yielding two-color spin-orbit entangled states with up to 97\% fidelity to the original one. Furthermore, our results show that the preservation of entanglement in HQFC requires the process to be driven by a classical field with non-separable degrees of freedom, showcasing a rare example where classical (local) non-separability is a necessary condition for quantum non-locality.