Spencer Weeden

Adaptive Spectroscopy of Fast Two-Level-System Dynamics in Superconducting Qubits

  1. Fabrizio Berritta,
  2. David Pahl,
  3. Lukas Pahl,
  4. William P. Banner,
  5. Gabriel Cutter,
  6. Jan A. Krzywda,
  7. Spencer Weeden,
  8. Shravan Patel,
  9. Paul Buttles,
  10. Stanislav Eilhart,
  11. Michael Gingras,
  12. Bethany M. Niedzielski,
  13. Robert McDermott,
  14. Mollie E. Schwartz,
  15. Kyle Serniak,
  16. Max Hays,
  17. Jeffrey A. Grover,
  18. and William D. Oliver
Parasitic two-level-system (TLS) defects are a major source of energy relaxation and temporal instability in superconducting quantum processors. Our sub-second adaptive spectroscopy
reveals telegraphic switching of TLSs with a characteristic timescale of a few seconds and spectral diffusion with diffusivity D≈0.9 MHz2/s. These timescales are about 3×102 times faster than what is observed in conventional nonadaptive spectroscopy, which typically requires hours of measurement time. We resolve such fast dynamics on a field-programmable gate array (FPGA)-based controller that enables measurement of frequency- and time-resolved relaxations with sub-second temporal resolution in flux-tunable superconducting qubits. We observe similar defect dynamics across multiple qubits in independently fabricated devices measured in different laboratories. We correlate TLS-induced fluctuations with gate-level errors using randomized benchmarking. Our results reveal a previously inaccessible regime of frequency-resolved TLS dynamics and redefine the timescales relevant to TLS-aware characterization and calibration of superconducting quantum processors.
arxiv pdf

Scaffold-Assisted Window Junctions for Superconducting Qubit Fabrication

  1. Chung-Ting Ke,
  2. Jun-Yi Tsai,
  3. Yen-Chun Chen,
  4. Zhen-Wei Xu,
  5. Elam Blackwell,
  6. Matthew A. Snyder,
  7. Spencer Weeden,
  8. Peng-Sheng Chen,
  9. Chih-Ming Lai,
  10. Shyh-Shyuan Sheu,
  11. Zihao Yang,
  12. Cen-Shawn Wu,
  13. Alan Ho,
  14. R. McDermott,
  15. John Martinis,
  16. and Chii-Dong Chen
The superconducting qubit is one of the promising directions in realizing fault-tolerant quantum computing (FTQC), which requires many high-quality qubits. To achieve this, it is desirable
to leverage modern semiconductor industry technology to ensure quality, uniformity, and reproducibility. However, conventional Josephson junction fabrication relies mainly on resist-assistant double-angle evaporation, posing integration challenges. Here, we demonstrate a lift-off-free qubit fabrication that integrates seamlessly with existing industrial technologies. This method employs a silicon oxide (SiO2) scaffold to define an etched window with a well-controlled size to form a Josephson junction. The SiO2, which has a large dielectric loss, is etched away in the final step using vapor HF leaving little residue. This Window junction (WJ) process mitigates the degradation of qubit quality during fabrication and allows clean removal of the scaffold. The WJ process is validated by inspection and Josephson junction measurement. The scaffold removal process is verified by measuring the quality factor of the resonators. Furthermore, compared to scaffolds fabricated by plasma-enhanced chemical vapor deposition (PECVD), qubits made by WJ through physical vapor deposition (PVD) achieve relaxation time up to 57μs. Our results pave the way for a lift-off-free qubit fabrication process, designed to be compatible with modern foundry tools and capable of minimizing damage to the substrate and material surfaces.
arxiv pdf