Reliable small-gap capacitors in an electro-opto-mechanical transducer*

Bidirectional microwave-to-optical frequency transducers will allow superconducting qubit nodes to be connected via networks of fiber optic cables. Our electro-optic transducer uses a high-Q mechanical mode of a Si₃N₄ membrane to simultaneously modulate the resonances of a vacuum-gap, superconducting LC circuit and a Fabry-Perot optical cavity. Our device operates with high efficiency and low added noise within a narrow bandwidth, limited by microwave-induced excess loss and noise [1]. Decreasing the capacitor-gap spacing in a controlled manner would increase the electromechanical coupling. We present an upgraded flip-chip design that improves electromechanical coupling and device reliability by rigidly pinning the membrane on a node of motion. Crucially, we demonstrate that this does not degrade our mechanical Q.

[1] B. M. Brubaker, J. M. Kindem, M. D. Urmey, S. Mittal, R. D. Delaney, P. S. Burns, M. R. Vissers, K. W. Lehnert, and C. A. Regal. Phys. Rev. X, 12.2 021062 (2022).