Xiqiao Wang

Precision frequency tuning of tunable transmon qubits using alternating-bias assisted annealing

  1. Xiqiao Wang,
  2. Joel Howard,
  3. Eyob A. Sete,
  4. Greg Stiehl,
  5. Cameron Kopas,
  6. Stefano Poletto,
  7. Xian Wu,
  8. Mark Field,
  9. Nicholas Sharac,
  10. Christopher Eckberg,
  11. Hilal Cansizoglu,
  12. Raja Katta,
  13. Josh Mutus,
  14. Andrew Bestwick,
  15. Kameshwar Yadavalli,
  16. and David P. Pappas
Superconducting quantum processors are one of the leading platforms for realizing scalable fault-tolerant quantum computation (FTQC). The recent demonstration of post-fabrication tuning
of Josephson junctions using alternating-bias assisted annealing (ABAA) technique and a reduction in junction loss after ABAA illuminates a promising path towards precision tuning of qubit frequency while maintaining high coherence. Here, we demonstrate precision tuning of the maximum |0⟩→|1⟩ transition frequency (fmax01) of tunable transmon qubits by performing ABAA at room temperature using commercially available test equipment. We characterize the impact of junction relaxation and aging on resistance spread after tuning, and demonstrate a frequency equivalent tuning precision of 7.7 MHz (0.17%) based on targeted resistance tuning on hundreds of qubits, with a resistance tuning range up to 18.5%. Cryogenic measurements on tuned and untuned qubits show evidence of improved coherence after ABAA with no significant impact on tunability. Despite a small global offset, we show an empirical fmax01 tuning precision of 18.4 MHz by tuning a set of multi-qubit processors targeting their designed Hamiltonians. We experimentally characterize high-fidelity parametric resonance iSWAP gates on two ABAA-tuned 9-qubit processors with fidelity as high as 99.51±0.20%. On the best-performing device, we measured across the device a median fidelity of 99.22% and an average fidelity of 99.13±0.12%. Yield modeling analysis predicts high detuning-edge-yield using ABAA beyond the 1000-qubit scale. These results demonstrate the cutting-edge capability of frequency targeting using ABAA and open up a new avenue to systematically improving Hamiltonian targeting and optimization for scaling high-performance superconducting quantum processors.
arxiv pdf

Alternating Bias Assisted Annealing of Amorphous Oxide Tunnel Junctions

  1. David P. Pappas,
  2. Mark Field,
  3. Cameron Kopas,
  4. Joel A. Howard,
  5. Xiqiao Wang,
  6. Ella Lachman,
  7. Lin Zhou,
  8. Jinsu Oh,
  9. Kameshwar Yadavalli,
  10. Eyob A. Sete,
  11. Andrew Bestwick,
  12. Matthew J. Kramer,
  13. and Joshua Y. Mutus
We demonstrate a transformational technique for controllably tuning the electrical properties of fabricated thermally oxidized amorphous aluminum-oxide tunnel junctions. Using conventional
test equipment to apply an alternating bias to a heated tunnel barrier, giant increases in the room temperature resistance, greater than 70%, can be achieved. The rate of resistance change is shown to be strongly temperature-dependent, and is independent of junction size in the sub-micron regime. In order to measure their tunneling properties at mK temperatures, we characterized transmon qubit junctions treated with this alternating-bias assisted annealing (ABAA) technique. The measured frequencies follow the Ambegaokar-Baratoff relation between the shifted resistance and critical current. Further, these studies show a reduction of junction-contributed loss on the order of ≈2×10−6, along with a significant reduction in resonant- and off-resonant-two level system defects when compared to untreated samples. Imaging with high-resolution TEM shows that the barrier is still predominantly amorphous with a more uniform distribution of aluminum coordination across the barrier relative to untreated junctions. This new approach is expected to be widely applicable to a broad range of devices that rely on amorphous aluminum oxide, as well as the many other metal-insulator-metal structures used in modern electronics.
arxiv pdf