Investigating the impact of Nb₂O₅ on quantum coherence via selective oxygen scavenging

Implementation of superconducting qubit-based quantum processors is precluded prominently by the decoherence processes emerging from materials-inherent defects. The origin of these defects is yet to be understood, posing great urgency with the rise of quantum information science. The central question is the dielectric losses introduced by the formation of native oxides on Al and Nb used to build superconducting qubits. A recent study by our group on Nb oxidation indicates a strong impact of Nb suboxides on coherence times, where the microwave losses in qubits inversely scale with the thickness of the NbO_x on Nb devices. While oxide is considered a primary effect for this improvement, DFT calculations by Griffin et. al., shown that sub-stoichiometric Nb₂O₅ has a paramagnetic phase that can be a source of the decoherence. In this study, we investigated the impact of Nb₂O₅ on coherence time by selectively eliminating Nb₂O₅ via oxygen scavenging by TiN grown with atomic layer deposition. TiN passivated resonators show traces of Nb₂O₅ while NbO and NbO₂ persist on the Nb with native oxide according to photoemission spectroscopy. Selective reduction of Nb₂O₅ to NbO and mitigation of the associated defects suggests a viable step towards development of fault tolerant quantum processor.