A chip-scale source of entangled microwave and optical photonic qubits*

Superconducting microwave circuits and optical photons are leading platforms for quantum computation and communication, respectively. We demonstrate a chip-scale source of entangled microwave and optical photonic qubits. Our device features a piezo-optomechanical transducer integrated with a light-robust superconducting resonator and operated as a microwave-optical photon-pair source. Using two consecutive pump pulses, we drive photon-pair emission into dual-rail optical and microwave photonic qubits defined on 'early' and 'late' output modes, and prepare microwave-optical Bell states. Entanglement is verified by measuring microwave-optical correlations in two orthogonal bases. Such a device can be used for optical distribution of entanglement between superconducting qubits.