Cavity Piezomechanics for Microwave-Optical Quantum Transduction and Network*

Connecting the two quantum worlds in microwave and optics is a long-pursing goal in quantum information science. It would allow the integration of state-of-the-art microwave quantum processors, such as superconducting qubits, with long-distance quantum communication channels through fiber optics, hence establish a future quantum network. High-fidelity quantum state transfer between microwave and optical frequencies will also enable new quantum technologies such as distributed quantum computing and sensing. Here, we discuss microwave-optical quantum transduction with a focus on cavity piezomechanics as an integrated platform for providing efficient interaction between superconducting and nanophotonic circuits. Importantly, the high operation frequency of piezomechanics at several GHz can substantially suppress thermal noise, making it an appealing candidate for quantum applications. We present experimental efforts on integrated superconducting piezo-optomechanical transducers with simultaneous electromechanical and optomechanical cavity enhancement and demonstrate bidirectional linear photon conversion between microwave and optics. We also discuss the entanglement-based quantum transduction and compare with the direct quantum transduction scheme in the framework of quantum channel theory. Based on practical piezo-optomechanical system parameters, we show that the entanglement-based scheme can indeed admit a positive transduction rate when the direct quantum transduction has zero quantum capacity. Using a pair of piezo-optomechanical transducers, we also propose protocols for remote microwave-microwave entanglement within both continuous variable and discrete variable settings, showing the potential of connecting distant microwave quantum processors via quantum teleportation.