Nitrogen plasma passivated niobium resonators for superconducting quantum circuits

Microwave loss in niobium metallic structures used for superconducting quantum circuits is limited by a native surface oxide layer formed over timescale of minutes when exposed to an ambient environment. In this work, we show that nitrogen plasma treatment forms nitride at the metal–air interface, which prevents such oxidation. X-ray photoelectron spectroscopy confirms doping more than 5 nm into and suppressed oxygen presence. This passivation remains stable after aging 15 days Cryogenic microwave characterization shows average filling-factor-adjusted two-level-system tangent [Formula: see text] resonators with 3 text]m center strip 20 strip, exceeding performance unpassivated samples factor four.