Investigation of coherence of niobium-based resonators enabled by a fast-sealing microwave cavity

TL;DR

This work introduces a fast-sealing microwave cavity that significantly reduces metal-air interface loss in niobium resonators, enabling high coherence times and providing a platform for studying oxide regrowth.

Contribution

Development of a rapid sealing method to minimize interface loss in Nb superconducting devices, enhancing coherence and enabling controlled oxide regrowth studies.

Findings

Resonators sealed in the cavity achieve Q factors over one million at single-photon power.

Devices show frequency shifts and Q degradation after air exposure, consistent with Nb oxide regrowth.

The method provides a practical approach to sustain high coherence in Nb superconducting devices.

Abstract

Resonators and qubits with a niobium (Nb) base metal layer achieve some of the highest coherence times in superconducting quantum devices. The performance of such devices is often limited by loss associated with two-level systems, which are found primarily at material surfaces and interfaces. The metal-air (MA) interface is a major contributor to device loss. In this work, we develop a fast-sealing microwave cavity that enables devices to be placed under vacuum within five minutes of oxide removal, thereby significantly reducing the MA interface loss compared to common device processing and packaging approaches. Using coplanar stripline resonators, we demonstrate that devices sealed in such a cavity exhibit internal quality factors exceeding one million at single-photon power. After re-exposure to air, the devices show downward resonance frequency shifts and quality factor degradations, quantitatively consistent with a model of Nb oxide regrowth. The fast-sealing microwave cavity provides a practical and consistent method to mitigate MA interface loss and sustain high coherence in Nb devices, and establishes a controlled platform for studying metal oxide regrowth kinetics and dielectric properties, the understanding of which is critical to achieving high coherence in superconducting quantum devices.