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

TL;DR

This study explains why tantalum (Ta) superconducting qubits exhibit fewer tunneling two-level systems (TLS) than niobium (Nb) qubits, attributing it to differences in their oxide interfaces and atomic properties, which impact qubit coherence.

Contribution

The paper provides a detailed atomic-level analysis of Ta and Nb oxides, revealing how Ta's smoother surface and higher atomic mass reduce TLS, offering insights for improving qubit coherence.

Findings

Ta$_2$O$_5$ forms a smoother surface with fewer dangling O atoms than Nb$_2$O$_5$.

Ta's higher atomic mass lowers TLS tunnel splittings below qubit frequencies.

External electric fields or SO$_2$ passivation can further reduce TLS on Nb surfaces.

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.