Compact superconducting vacuum-gap capacitors with low microwave loss and high mechanical coherence for scalable quantum circuits

TL;DR

This paper introduces a scalable fabrication process for vacuum-gap superconducting capacitors that support ultra-high-coherence mechanical modes, achieving low microwave loss and enabling advanced quantum optomechanical experiments.

Contribution

The authors develop a scalable fabrication method for vacuum-gap capacitors with ultra-high mechanical coherence and low microwave loss, demonstrating ground-state cooling and quantum dynamics in mechanical systems.

Findings

Achieved vacuum gaps of approximately 150 nm using a planarized SiO₂ sacrificial layer.

Demonstrated mechanical quality factor of 40 million in ground-state cooled mechanical oscillators.

Realized an optomechanical topological lattice with 24 sites and observed quantum collective dynamics.

Abstract

Vacuum-gap capacitors have recently attracted significant interest in superconducting circuit platforms due to their compact design and exceptionally low dielectric losses in the microwave regime. Their intrinsic ability to support mechanical vibrational modes makes them well-suited for circuit optomechanics. However, precise control over the gap size and the realization of high-coherence mechanical modes remain longstanding challenges. Here, we present a detailed and scalable fabrication process for vacuum-gap capacitors that support ultra-high-coherence mechanical motion, exhibit low microwave loss, and occupy a significantly smaller footprint compared to conventional planar geometries. By employing a planarized $\mathrm{SiO}_2$ sacrificial layer, we achieve vacuum gaps on the order of 150 nm. Using this platform, we have recently demonstrated ground-state cooling and motion squeezing of a mechanical oscillator with a quality factor of 40 million, a 100-fold improvement compared to prior works, as well as a single-photon optomechanical coupling rate of approximately 15Hz. Additional achievements include the realization of an optomechanical topological lattice with 24 sites and the observation of quantum collective dynamics in a mechanical hexamer. Collectively, these results underscore the potential of vacuum-gap capacitors as a platform for coupling superconducting qubits to mechanical systems, enabling quantum storage, and probing gravitational effects in quantum mechanics.