Cryogenic rf-to-microwave transducer based on a dc-biased electromechanical system

We report a two-stage, heterodyne rf-to-microwave transducer that combines a tunable electrostatic pre-amplifier with a superconducting electromechanical cavity. A metalized Si₃N₄   membrane (3 MHz frequency) forms the movable plate of a vacuum-gap capacitor in a microwave LC resonator. A dc bias across the gap converts any small rf signal into a resonant electrostatic force proportional to the bias, providing a voltage-controlled gain that multiplies the cavity’s intrinsic electromechanical gain. In a flip-chip device with a 1.5 µm gap operated at 10 mK we observe dc-tunable anti-spring shifts, and rf-to-microwave transduction at 49 V bias, achieving a charge sensitivity of 87 µe/Hz^1/2   (0.9 nV/Hz^1/2). Extrapolation to sub-micron gaps and state-of-the-art Q>10⁸ membrane resonators predicts sub-200 fV/Hz^1/2 sensitivity, establishing dc-biased electromechanics as a practical route towards quantum-grade rf electrometers and low-noise modular heterodyne links for superconducting microwave circuits and charge or voltage sensing.

The preprint can be found on ArxiV: https://arxiv.org/abs/2508.01066

This work is a collaboration with Alice & Bob. It was supported by ANR: MecaFlux (ANR-21-CE47-0011). This work has also been supported by Region Île-de-France in the framework of DIM Sirteq (project CryoParis) and DIM QuanTiP (project COCONUT), and by Sorbonne Université through the HyQuTech “Emergence” program. It was also funded by the Quantum Information Center Sorbonne (QICS doctoral fellowship), and by the CNRS–University of Arizona joint Ph.D. program.