Thomas F. Harrelson

Microscopic Theory of Magnetic Disorder-Induced Decoherence in Superconducting Nb Films

  1. Evan Sheridan,
  2. Thomas F. Harrelson,
  3. Eric Sivonxay,
  4. Kristin A. Persson,
  5. M. Virginia P. Altoé,
  6. Irfan Siddiqi,
  7. D. Frank Ogletree,
  8. David I. Santiago,
  9. and Sinéad M. Griffin
The performance of superconducting qubits is orders of magnitude below what is expected from theoretical estimates based on the loss tangents of the constituent bulk materials. This
has been attributed to the presence of uncontrolled surface oxides formed during fabrication which can introduce defects and impurities that create decoherence channels. Here, we develop an ab initio Shiba theory to investigate the microscopic origin of magnetic-induced decoherence in niobium thin film superconductors and the formation of native oxides. Our ab initio calculations encompass the roles of structural disorder, stoichiometry, and strain on the formation of decoherence-inducing local spin moments. With parameters derived from these first-principles calculations we develop an effective quasi-classical model of magnetic-induced losses in the superconductor. We identify d-channel losses (associated with oxygen vacancies) as especially parasitic, resulting in a residual zero temperature surface impedance. This work provides a route to connecting atomic scale properties of superconducting materials and macroscopic decoherence channels affecting quantum systems.
arxiv pdf

Elucidating the local atomic and electronic structure of amorphous oxidized superconducting niobium films

  1. Thomas F. Harrelson,
  2. Evan Sheridan,
  3. Ellis Kennedy,
  4. John Vinson,
  5. Alpha T. N'Diaye,
  6. M. Virginia P. Altoé,
  7. Adam Schwartzberg,
  8. Irfan Siddiqi,
  9. D. Frank Ogletree,
  10. Mary C. Scott,
  11. and Sinéad M. Griffin
Qubits made from superconducting materials are a mature platform for quantum information science application such as quantum computing. However, materials-based losses are now a limiting
factor in reaching the coherence times needed for applications. In particular, knowledge of the atomistic structure and properties of the circuit materials is needed to identify, understand, and mitigate materials-based decoherence channels. In this work we characterize the atomic structure of the native oxide film formed on Nb resonators by comparing fluctuation electron microscopy experiments to density functional theory calculations, finding that an amorphous layer consistent with an Nb2O5 stoichiometry. Comparing X-ray absorption measurements at the Oxygen K edge with first-principles calculations, we find evidence of d-type magnetic impurities in our sample, known to cause impedance in proximal superconductors. This work identifies the structural and chemical composition of the oxide layer grown on Nb superconductors, and shows that soft X-ray absorption can fingerprint magnetic impurities in these superconducting systems.
arxiv pdf