Characterization of fabrication methods to reach high coherence superconducting quantum circuits*

The fabrication of superconducting qubits and resonators with long coherence times and high quality factors is an important milestone on the way towards useful quantum applications. Although significant improvements in coherence time have been achieved over the last years, reaching qubit lifetimes well beyond 100 µs involves careful investigation of all fabrication steps. Here, we demonstrate that such high device qualities can be achieved by a combination of substrate cleaning, etching optimization and post-process sample cleaning. Thereby, we reach quality factors well above 1x10^7 for thin-film niobium CPW resonators and can observe transmon qubits with more than 700 µs lifetime. In addition, we exploit the high quality of the niobium resonators to investigate losses arising from different types of silicon substrates.