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Abstract
Superconducting qubits are a key contender for quantum computing elements, but they often face challenges like noise and decoherence from two-level systems (TLS). Tantalum (Ta) qubits are notable for their long T$_1$ coherence times nearing milliseconds, mainly due to fewer TLS, though the cause was unclear. Our research explored this by analyzing the air-oxide interface with density functional theory, particularly comparing Nb oxide (Nb$_2$O$_5$) and Ta oxide (Ta$_2$O$_5$). We discovered that Ta$_2$O$_5$ forms a smoother surface with fewer dangling O atoms and TLS than Nb$_2$O$_5$. The greater atomic mass of Ta also lowers the TLS tunnel splittings below the qubit's operating frequency. Furthermore, using external electric fields or SO$_2$ passivation can significantly reduce TLS on Nb surfaces, potentially improving their coherence times.