  Entanglement is an extraordinary feature of quantum mechanics. Sources of
entangled optical photons were essential to test the foundations of quantum
physics through violations of Bell's inequalities. More recently, entangled
many-body states have been realized via strong non-linear interactions in
microwave circuits with superconducting qubits. Here we demonstrate a
chip-scale source of entangled optical and microwave photonic qubits. Our
device platform integrates a piezo-optomechanical transducer with a
superconducting resonator which is robust under optical illumination. We drive
a photon-pair generation process and employ a dual-rail encoding intrinsic to
our system to prepare entangled states of microwave and optical photons. We
place a lower bound on the fidelity of the entangled state by measuring
microwave and optical photons in two orthogonal bases. This entanglement source
can directly interface telecom wavelength time-bin qubits and GHz frequency
superconducting qubits, two well-established platforms for quantum
communication and computation, respectively.
