Continuous variable entanglement between microwave and optics

Entanglement enables quantum advantage over classical applications. Entanglement between similar systems such as photons, ions, atoms and nuclear spins has already shown this advantage in information processing, communication, cryptography and sensing. Moreover, hybrid entanglement between localized systems and itinerant photons has extended these applications to distributed quantum computing and sensing. Thus far, such hybrid entanglement has remained divided into two paradigms - microwave photons entangled with microwave-based quantum devices such as superconducting qubits, and optical photons entangled with optically-addressable systems such as atoms. Uniting these two paradigms will enable new capabilities in hybrid quantum networks, sensing and meteorology. The required entanglement between itinerant microwave and optical light has not been demonstrated due to the incompatibility of low loss superconductivity and high energy optical photons which prevented the required ultra-low noise conditions. Here, we demonstrate the deterministic preparation of an entangled microwave-optical state in the continuous variable domain that is squeezed 0.7 dB below the vacuum level. We achieve this in a triply resonant, pulsed electro-optic interconnect working in a millikelvin environment.