Qualifying transmon qubits using dc transport measurements

Low noise dc electrical transport measurements provide a reliable means of characterizing the quality of the Josephson junctions (JJs) that form the heart of superconducting qubits. Measurements of the current-voltage (IV) characteristics can provide a direct readout of important junction parameters such as the Josephson energy Ej and the charging energy Ec. Here we report our investigation of an unexpected feature in the IV characteristics: the appearance of a finite resistance at currents below the critical current, where the resistance of the junction is expected to vanish. This "zero bias" resistance is observed in all the JJs that we have measured, fabricated by four different groups in the SQMS collaboration. The origin of this zero bias resistance and its potential implications for qubit performance will be discussed.