Why Superconducting Ta Qubits Have Fewer Tunneling Two-Level Systems at the Vacuum-Oxide Interface Than Nb Qubits

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 T₁ coherence times nearing milliseconds, presumably due to fewer TLS, though the cause was unclear. Our research explored this by analyzing the vacuum-oxide interface with density functional theory, particularly comparing Nb oxide (Nb₂O₅) and Ta oxide (Ta₂O₅). We discovered that Ta₂O₅ forms a smoother surface with fewer dangling O atoms and TLS than Nb₂O₅. The greater atomic mass of Ta also lowers the TLS tunnel splittings below the qubit's operating frequency. Furthermore, using external electric fields or SO₂ passivation can significantly reduce TLS on Nb surfaces, potentially improving their coherence times.