Parasitic Losses in Nb Superconducting Resonators

Microwave losses in niobium (Nb) superconducting resonators are investigated at milli-Kelvin temperatures and with low drive power. In addition to the well-known suppression in Q-factor that arises from coupling between the resonator and two-level defects in the dielectric substrate, we report strong dependence of the measured Q-factor and resonance line-shape on the electromagnetic environment. Methods to suppress parasitic coupling between the resonator and its environment are discussed.