Vacuum-Gap Capacitors for Low-Loss Superconducting Resonant Circuits

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 as4× 10⁻⁵, significantly lower than with capacitors using amorphous dielectrics. VGCs are structurally robust, small, and easily scaled to capacitance values above 100 pF.