Characterizations of Superconducting Ta Films for Quantum Computing

Recently, a breakthrough was reported in the long coherence time (> 0.5 ms) transmon qubit using tantalum films as the base superconductor in the devices. Here we report a study of superconducting properties in Ta films showing a similar normal state resistive behavior and superconducting transition temperature T_C. Using noncontact ac magnetic susceptibility measurements, we found strikingly different response in their imaginary part of magnetic susceptibility χ''. A sharp peak and near zero loss in χ′′ below T_C were observed in some films, indicating a strongly coupled superconductor. A broad peak and massive loss in χ′′ below T_C were observed in another films, that could be a leading source of decoherence. This study demonstrates that the ac susceptibility is an excellent method for characterizing bulk superconducting properties of the constituent materials used in qubits.