Hilal Cansizoglu

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

Systematic Improvements in Transmon Qubit Coherence Enabled by Niobium Surface Encapsulation

  1. Mustafa Bal,
  2. Akshay A. Murthy,
  3. Shaojiang Zhu,
  4. Francesco Crisa,
  5. Xinyuan You,
  6. Ziwen Huang,
  7. Tanay Roy,
  8. Jaeyel Lee,
  9. David van Zanten,
  10. Roman Pilipenko,
  11. Ivan Nekrashevich,
  12. Daniel Bafia,
  13. Yulia Krasnikova,
  14. Cameron J. Kopas,
  15. Ella O. Lachman,
  16. Duncan Miller,
  17. Josh Y. Mutus,
  18. Matthew J. Reagor,
  19. Hilal Cansizoglu,
  20. Jayss Marshall,
  21. David P. Pappas,
  22. Kim Vu,
  23. Kameshwar Yadavalli,
  24. Jin-Su Oh,
  25. Lin Zhou,
  26. Matthew J. Kramer,
  27. Dominic P. Goronzy,
  28. Carlos G. Torres-Castanedo,
  29. Graham Pritchard,
  30. Vinayak P. Dravid,
  31. James M. Rondinelli,
  32. Michael J. Bedzyk,
  33. Mark C. Hersam,
  34. John Zasadzinski,
  35. Jens Koch,
  36. James A. Sauls,
  37. Alexander Romanenko,
  38. and Anna Grassellino
We present a novel transmon qubit fabrication technique that yields systematic improvements in T1 coherence times. We fabricate devices using an encapsulation strategy that involves
passivating the surface of niobium and thereby preventing the formation of its lossy surface oxide. By maintaining the same superconducting metal and only varying the surface structure, this comparative investigation examining different capping materials and film substrates across different qubit foundries definitively demonstrates the detrimental impact that niobium oxides have on the coherence times of superconducting qubits, compared to native oxides of tantalum, aluminum or titanium nitride. Our surface-encapsulated niobium qubit devices exhibit T1 coherence times 2 to 5 times longer than baseline niobium qubit devices with native niobium oxides. When capping niobium with tantalum, we obtain median qubit lifetimes above 200 microseconds. Our comparative structural and chemical analysis suggests that amorphous niobium suboxides may induce higher losses. These results are in line with high-accuracy measurements of the niobium oxide loss tangent obtained with ultra-high Q superconducting radiofrequency (SRF) cavities. This new surface encapsulation strategy enables further reduction of dielectric losses via passivation with ambient-stable materials, while preserving fabrication and scalable manufacturability thanks to the compatibility with silicon processes.
arxiv pdf