Direct Measurement of Microwave Loss in Nb Films for Superconducting Qubits

TL;DR

This study directly measures microwave losses in niobium films used for superconducting qubits, revealing that these losses are comparable to high-quality bulk niobium and are not the main factor limiting qubit coherence.

Contribution

First direct correlation of RF losses in niobium films with material parameters using a 3D SRF resonator, isolating film loss from other contributions.

Findings

Niobium film microwave dissipation resembles high-quality bulk niobium SRF cavities.

Microstructure and impurities do not significantly affect niobium film losses.

Dielectric oxide, not niobium loss, limits qubit coherence.

Abstract

Niobium films are a key component in modern two-dimensional superconducting qubits, yet their contribution to the total qubit decay rate is not fully understood. The presence of different layers of materials and interfaces makes it difficult to identify the dominant loss channels in present two-dimensional qubit designs. In this paper we present the first study which directly correlates measurements of RF losses in such films to material parameters by investigating a high-power impulse magnetron sputtered (HiPIMS) film atop a three-dimensional niobium superconducting radiofrequency (SRF) resonator. By using a 3D SRF structure, we are able to isolate the niobium film loss from other contributions. Our findings indicate that microwave dissipation in the HiPIMS-prepared niobium films, within the quantum regime, resembles that of record-high intrinsic quality factor of bulk niobium SRF cavities, with lifetimes extending into seconds. Microstructure and impurity level of the niobium film do not significantly affect the losses. These results set the scale of microwave losses in niobium films and show that niobium losses do not dominate the observed coherence times in present two-dimensional superconducting qubit designs, instead highlighting the dominant role of the dielectric oxide in limiting the performance. We can also set a bound for when niobium film losses will become a limitation for qubit lifetimes.