Two-colour high-purity Einstein-Podolsky-Rosen photonic state.

Entanglement is the backbone of quantum information science and its applications. Entangled states of light are necessary for distributed quantum protocols, quantum sensing and quantum internet. A distributed quantum network requires entanglement between light modes of different colours optimized for interaction with the nodes as well as for communication between them. Here we demonstrate a high-purity Einstein-Podolsky-Rosen (EPR) entangled state between light modes with the wavelengths separated by more than 200 nm. The modes display $-7.7\pm0.5$ dB of two-mode entanglement and an overall state purity of $0.63\pm0.16$. Entanglement is observed over five octaves of sideband frequencies from rf down to audio-band. In the context of two-colour entanglement, the demonstrated combination of high state purity, strong entanglement, and extended frequency range paves the way to new matter-light quantum protocols, such as teleportation between disparate quantum systems, quantum sensing and quantum-enhanced gravitational wave interferometry. The scheme demonstrated here can be readily applied towards entanglement between telecom wavelengths and atomic quantum memories.