Microwave Loss Characteristics of Nb Films for Quantum Computing

Niobium films typically constitute a significant component in the construction of modern two-dimensional superconducting qubit architectures, yet their contribution to the total decay rate is not fully understood, making it difficult to definitively identify dominant loss channels in present designs. We present the first study, which correlates direct 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. 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 role of the dielectric oxide in limiting the performance.