Chengyu Song

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

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