Direct Measurement of Microwave Loss in Nb Films for Superconducting Qubits
The Science: 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. Losses associated with the Nb film and the metal-air interface have been directly measured and quantified experimentally.
The Impact: Distinguishing loss mechanisms in qubit performance is challenging, and theoretical calculations or simulations may not be as reliable as experimental measurements. A 3D SRF cavity can isolate the Nb thin film, enabling direct measurement of conductive losses in Nb and TLS losses at the metal-air interface.
Summary: The presence of different layers of materials and interfaces makes it difficult to identify the dominant loss channels in present two-dimensional qubit designs. This work 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.
Contact: Bektur Abdisatarov bektur@fnal.gov
Focus area: Quantum Materials
Institutions: Fermi National Accelerator Laboratory
Citation: B. Abdisatarov et al., “Direct measurement of microwave loss in Nb films for superconducting qubits,” Applied Physics Letters 125, 124002, 2024.
Funding acknowledgment: This material is based upon work supported by the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers, Superconducting Quantum Materials and Systems Center (SQMS) under contract number DE-AC02-07CH11359.