A Josephson junction~resonator to test~the quality of superconducting qubit circuits

Superconducting Josephson junction~resonators can~probe nonlinear oscillators~such~as~qubit~readout amplifiers and~qubits themselves.~We have fabricated~weakly coupled resonators~with a~flux tunable resonant frequency of over 1 GHz.~ At high powers the Josephson junction~resonators become nonlinear and two stable oscillation states are observed which can be harnessed to readout qubits.~ At sufficiently low powers, the~resonators~can probe a qubit since they store no more than~one photon of energy.~~At these low powers we observe two-level system defects attributed to the Josephson junction,~similar to those observed in the phase qubit.~ We have fabricated and measured~resonators with different~device parameters, such as the junction area and the critical current density.~ We will report on~the quality of the~resonators as a function of the device parameters.