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

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.

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Cited by in corpus (2)

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