Robust design and assembly of a phononically-shielded electro-mechanical-optical converter*

A bidirectional, quantum-coherent microwave-to-optical converter is a long-standing technological goal that would enable disparate quantum computational nodes based on superconducting circuits to be interconnected with optical fibers. We construct such a converter by simultaneously coupling a mechanical mode of a SiN membrane to superconducting LC circuit and optical Fabry-Perot cavity. Even at millikelvin temperatures, a high mechanical quality factor is crucial to reducing the added noise due to the MHz scale frequency of the membrane. We have implemented a phononic shield that isolates the membrane from its thermal environment within the complex requirements of the electro-optic circuit, and have achieved a mechanical Q of greater than 12 million. Additionally, we will present a new flip chip construction method that provides reliable electromechanical coupling without sacrificing the improvements to the mechanical Q.