John Zasadzinski

Microwave dielectric properties of LiNbO3 and AlN at millikelvin temperatures and single-photon power

  1. Alessandro Reineri,
  2. Francesco Crisa,
  3. Akshay Murthy,
  4. Maithlee Shinde,
  5. Daniel Bafia,
  6. Changqing Wang,
  7. Tanay Roy,
  8. Alexander Romanenko,
  9. John Zasadzinski,
  10. Anna Grassellino,
  11. and Silvia Zorzetti
Efficient bidirectional microwave optical photon conversion is a key capability for scaling superconducting quantum processors into distributed networks. However, achieving the necessary
conversion efficiency requires filling a critical knowledge gap in understanding the loss mechanisms of electro optic materials. Here, we characterize the microwave properties of single crystal bulk LiNbO3 and AlN over a broad range of powers, down to single photon levels, and spanning from millikelvin temperatures to above 1K. We demonstrate that both materials exhibit two level systems (TLS) behavior, while piezoelectric related losses are excluded. We show that TLS induced dissipation is predominantly localized on the surface rather than being an intrinsic bulk property, a result further corroborated by room temperature 3D XPS and time of flight SIMS analyses. These findings provide useful insights to engineer hybrid architectures that integrate bulk electro optic crystals within superconducting cavities, proving that microwave quality factors compatible with high efficiency microwave optical transduction are within reach.
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