Reducing Microwave Losses in Niobium Thin Films for Superconducting Quantum Devices*

Superconducting quantum devices have gained prominence in quantum computing systems, with niobium (Nb) thin films serving as a necessary component. Understanding and mitigating losses in such systems is a complex challenge, as various loss mechanisms can influence qubit performance. To isolate and address the intrinsic loss associated with Nb thin films, we conducted a study involving the deposition of Nb films onto the inner surface of a bulk Nb cavity. Our investigation focused on microwave dissipation at cryogenic temperatures. Microwave measurements unveiled the intrinsic loss associated with the Nb thin film is comparable with the losses observed in bulk Nb. In our ongoing quest to minimize losses, we undertook a comprehensive analysis of the microstructure, chemical composition, and superconducting properties of Nb thin films deposited on Nb substrates under the same conditions as the Nb cavity. Annealing treatments applied to these samples effectively dissolved surface oxides and significantly reduced microwave losses. Our research explored various annealing techniques, with a specific focus on the performance of the Nb thin film at quantum regime. By investigating loss mechanisms, including two-level system losses, we observed a marked reduction in losses following baking treatments. These promising results provide valuable insights into the potential for optimizing the quality of Nb thin films in order to increase the lifetime of the quantum states superconducting quantum devices.