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Why superconducting Ta qubits have fewer tunneling two-level systems at the vacuum-oxide interface than Nb qubits

Zhe Wang, Clare C. Yu, and Ruqian Wu*

  • *Contact author: wur@uci.edu
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Phys. Rev. Applied 23, 024017 – Published 6 February, 2025

DOI: https://doi.org/10.1103/PhysRevApplied.23.024017

Abstract

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 (Ta) qubits are notable for their long T1 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 Nb oxide (Nb2O5) and Ta oxide (Ta2O5). We discover that Ta2O5 forms a smoother surface with fewer dangling O atoms and structural TLSs than Nb2O5. The greater atomic mass of Ta also lowers the tunnel splittings of these structural TLSs below the qubit’s operating frequency. Furthermore, using external electric fields or SO2 passivation can significantly reduce defect-related TLSs on Nb surfaces, potentially improving their coherence times.

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Phys. Rev. Applied 23, 024017– Published 6 February, 2025
  • Received 8 April 2024
  • Revised 10 January 2025
  • Accepted 13 January 2025
Article part of CHORUS
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