Adam Schwartzberg

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

Localization and reduction of superconducting quantum coherent circuit losses

  1. M. Virginia P. Altoé,
  2. Archan Banerjee,
  3. Cassidy Berk,
  4. Ahmed Hajr,
  5. Adam Schwartzberg,
  6. Chengyu Song,
  7. Mohammed Al Ghadeer,
  8. Shaul Aloni,
  9. Michael J. Elowson,
  10. John Mark Kreikebaum,
  11. Ed K. Wong,
  12. Sinead Griffin,
  13. Saleem Rao,
  14. Alexander Weber-Bargioni,
  15. Andrew M. Minor,
  16. David I. Santiago,
  17. Stefano Cabrini,
  18. Irfan Siddiqi,
  19. and D. Frank Ogletree
Quantum sensing and computation can be realized with superconducting microwave circuits. Qubits are engineered quantum systems of capacitors and inductors with non-linear Josephson
junctions. They operate in the single-excitation quantum regime, photons of 27μeV at 6.5 GHz. Quantum coherence is fundamentally limited by materials defects, in particular atomic-scale parasitic two-level systems (TLS) in amorphous dielectrics at circuit interfaces.[1] The electric fields driving oscillating charges in quantum circuits resonantly couple to TLS, producing phase noise and dissipation. We use coplanar niobium-on-silicon superconducting resonators to probe decoherence in quantum circuits. By selectively modifying interface dielectrics, we show that most TLS losses come from the silicon surface oxide, and most non-TLS losses are distributed throughout the niobium surface oxide. Through post-fabrication interface modification we reduced TLS losses by 85% and non-TLS losses by 72%, obtaining record single-photon resonator quality factors above 5 million and approaching a regime where non-TLS losses are dominant. [1]Müller, C., Cole, J. H. & Lisenfeld, J. Towards understanding two-level-systems in amorphous solids: insights from quantum circuits. Rep. Prog. Phys. 82, 124501 (2019)
arxiv pdf