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.

尽管越来越多的证据表明材料缺陷是超导量子位实际应用的主要障碍，但人们对微观材料特性与量子位相干性之间的联系知之甚少。在这里，我们将 transmon 量子位弛豫时间 ( T ₁ ) 的测量与量子位制造中使用的多晶铌薄膜的光谱学和显微术相结合。通过比较使用三种不同技术沉积的薄膜，我们揭示了T ₁和固有的薄膜特性，如晶粒尺寸、沿晶界增强的氧扩散以及表面附近低价氧化物的浓度。量子位和谐振器测量显示了两级系统缺陷的特征，我们建议将其置于晶界和表面氧化物中。我们还表明，多晶铌薄膜的残余电阻率可用作量子位寿命的品质因数。这种理解量子位退相干的综合方法为材料驱动的超导量子位性能改进绘制了一条途径。