D. Frank Ogletree

Effects of Laser-Annealing on Fixed-Frequency Superconducting Qubits

  1. Hyunseong Kim,
  2. Christian Jünger,
  3. Alexis Morvan,
  4. Edward S. Barnard,
  5. William P. Livingston,
  6. M. Virginia P. Altoé,
  7. Yosep Kim,
  8. Chengyu Song,
  9. Larry Chen,
  10. John Mark Kreikebaum,
  11. D. Frank Ogletree,
  12. David I. Santiago,
  13. and Irfan Siddiqi
As superconducting quantum processors increase in complexity, techniques to overcome constraints on frequency crowding are needed. The recently developed method of laser-annealing provides
an effective post-fabrication method to adjust the frequency of superconducting qubits. Here, we present an automated laser-annealing apparatus based on conventional microscopy components and demonstrate preservation of highly coherent transmons. In one case, we observe a two-fold increase in coherence after laser-annealing and perform noise spectroscopy on this qubit to investigate the change in defect features, in particular two-level system defects. Finally, we present a local heating model as well as demonstrate aging stability for laser-annealing on the wafer scale. Our work constitutes an important first step towards both understanding the underlying physical mechanism and scaling up laser-annealing of superconducting qubits.
arxiv pdf

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

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