Investigation of Microwave Loss Induced by Oxide Regrowth in High- Q Niobium Resonators

To improve the performance of state-of-the-art superconducting quantum devices, microwave loss due to defects at amorphous interfacial layers must be reduced, via proper surface treatment. The authors study niobium resonators after removing native oxides by HF etching, which reduces losses about tenfold and yields a quality factor of 7×10⁶ in the single-photon limit. Losses reappear as oxides form upon exposure to air; Nb₂O₅ is the only surface oxide that grows significantly in the first week. These findings are of interest for a panoply of devices, inluding superconducting qubits, quantum-limited amplifiers, microwave kinetic-inductance detectors, and single-photon detectors.