Synchrotron X-ray studies of superconducting qubit materials

Despite dramatic improvements in the coherence times of superconducting qubits over the past decade, further progress is still needed to push quantum computers towards reality. Recent breakthroughs highlighted the paramount role played by the defect-prone native surface oxide layers in limiting the coherence of qubits. However, very little is known about the defects and their mechanism of coupling to the qubit degree of freedom, in part due to the lack of suitable probes for defects in thin amorphous layers.
Here, we report on the combined use of synchrotron X-ray scattering and spectroscopic tools to probe the surface electronic and bulk structural properties of Nb thin films deposited on sapphire, which emulate qubit capacitor pads. The thin films were deposited using four different techniques yielding varying film qualities and densities. Material properties, including crystalline grain sizes and surface oxide layer thicknesses and compositions, will be discussed in light of the coherence times obtained for transmon qubits using Nb capacitor pads deposited with the same four techniques. We will discuss possible scenarios for the correlations between the material properties and the qubit performance.