Vacuum-Gap Capacitors for Low-Loss Superconducting Resonant Circuits

TL;DR

This paper introduces vacuum-gap capacitors for superconducting circuits, significantly reducing energy loss caused by dielectric defects, and demonstrates their robustness, scalability, and low loss at cryogenic temperatures.

Contribution

The authors developed vacuum-gap capacitors using optical lithography and micromachining, achieving lower loss tangents than traditional amorphous dielectric capacitors in superconducting circuits.

Findings

Loss tangent as low as 4x10^{-5} at 50 mK

Vacuum-gap capacitors are structurally robust and scalable

Capacitance values above 100 pF are achievable

Abstract

Low-loss microwave components are used in many superconducting resonant circuits from multiplexed readouts of low-temperature detector arrays to quantum bits. Two-level system defects in amorphous dielectric materials cause excess energy loss. In an effort to improve capacitor components, we have used optical lithography and micromachining techniques to develop superconducting parallel-plate capacitors in which lossy dielectrics are replaced by vacuum gaps. Resonance measurements at 50 mK on lumped LC circuits that incorporate these vacuum-gap capacitors (VGCs) reveal loss tangents at low powers as low as 4x10^{-5}, significantly lower than with capacitors using amorphous dielectrics. VGCs are structurally robust, small, and easily scaled to capacitance values above 100 pF.