Niobium and Tantalum High Q Resonators for Photon Detectors

We have measured the quality factors and phase noise of niobium and tantalum coplanar waveguide microwave resonators on silicon. The results of both materials are similar. We reach quality factors up to 105. At low temperatures the quality factors show an anomalous increase, while the resonance frequency remains constant for increasing power levels. The resonance frequency starts to decrease at temperatures around a tenth of the critical temperature. The phase noise exhibits a 1/f like slope. We attribute this behavior to the silicon dielectric.