Abstract
Despite mounting evidence that materials imperfections are a major obstacle to practical applications of superconducting qubits, connections between microscopic material properties and qubit coherence are poorly understood. Here, we combine measurements of transmon qubit relaxation times (T1) with spectroscopy and microscopy of the polycrystalline niobium films used in qubit fabrication. By comparing films deposited using three different techniques, we reveal correlations between T1 and intrinsic film properties such as grain size, enhanced oxygen diffusion along grain boundaries, and the concentration of suboxides near the surface. Qubit and resonator measurements show signatures of two-level system defects, which we propose to be hosted in the grain boundaries and surface oxides. We also show that the residual resistance ratio of the polycrystalline niobium films can be used as a figure of merit for qubit lifetime. This comprehensive approach to understanding qubit decoherence charts a pathway for materials-driven improvements of superconducting qubit performance.
Highlights
Despite mounting evidence that materials imperfections are a major obstacle to practical applications of superconducting qubits, connections between microscopic material properties and qubit coherence are poorly understood
Qubit characterization was performed on transmon qubits, which are widely used for quantum computing and quantum simulation applications[1,10,11,24,25,26,27,28]
We study two variations of high-power impulse magnetron sputtering (HiPIMS), where the direct current (DC) ionization voltage is replaced with short, highpower pulses, potentially leading to a higher degree of ionization and denser films[30,31,32]
Summary
Despite mounting evidence that materials imperfections are a major obstacle to practical applications of superconducting qubits, connections between microscopic material properties and qubit coherence are poorly understood. We show that the residual resistance ratio of the polycrystalline niobium films can be used as a figure of merit for qubit lifetime This comprehensive approach to understanding qubit decoherence charts a pathway for materials-driven improvements of superconducting qubit performance. We compared three different deposition methods for the Nb films, yielding a quantifiable variation in the nanostructure, surface oxide composition, and transport properties Several of these film properties correlate with the measured qubit relaxation time T1. Our results are consistent with a significant role for TLS-induced dissipation due to defects hosted at grain boundaries and in the suboxides near the surface These results are a critical first step in connecting precise materials properties with microscopic models for decoherence and establishing a materials-based approach to enhance qubit performance
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