Why superconducting <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><mml:mi>Ta</mml:mi></mml:math> qubits have fewer tunneling two-level systems at the vacuum-oxide interface than <mml:math xmlns:mml="http:

Superconducting qubits are a key contender for quantum computing elements, but they often face challenges like noise and decoherence from two-level systems (TLSs). Tantalum ($\mathrm{Ta}$) qubits are notable for their long ${T}_{1}$ coherence times, nearing milliseconds, presumably due to fewer TLSs, though the cause of this is unclear. We explore this by analyzing the vacuum-oxide interface using density functional theory, particularly comparing $\mathrm{Nb}$ oxide (${\mathrm{Nb}}_{2}{\mathrm{O}}_{5}$) and $\mathrm{Ta}$ oxide (${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$). We discover that ${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$ forms a smoother surface with fewer dangling $\mathrm{O}$ atoms and structural TLSs than ${\mathrm{Nb}}_{2}{\mathrm{O}}_{5}$. The greater atomic mass of $\mathrm{Ta}$ also lowers the tunnel splittings of these structural TLSs below the qubit's operating frequency. Furthermore, using external electric fields or ${\mathrm{SO}}_{2}$ passivation can significantly reduce defect-related TLSs on $\mathrm{Nb}$ surfaces, potentially improving their coherence times.