James M. Rondinelli

Microstructural Topology as a Prescriptor for Quantum Coherence: Towards A Unified Framework for Decoherence in Superconducting Qubits

  1. Vinayak P. Dravid,
  2. Akshay A. Murthy,
  3. Peter Lim,
  4. Gabriel T. dos Santos,
  5. Ramandeep Mandia,
  6. James M. Rondinelli,
  7. Mark C. Hersam,
  8. and Roberto dos Reis
In superconducting quantum circuits, decoherence improvements are frequently obtained through process interventions that simultaneously modify surface chemistry, microstructural topology,
and device geometry, leaving mechanistic attribution structurally underdetermined. Predictive materials engineering requires measurable structural statistics to be separated from geometry-dependent coupling coefficients into independently testable factors. We introduce the concept of classical and quantum microstructure. In that context, we formulate a channel-wise separable framework for decoherence in superconducting transmon qubits in which each loss channel is described by a reduced prescriptor. Here, a channel-specific microstructural state variable is determined independently of device geometry, and a geometry-dependent coupling functional is computable from field solutions without reference to surface chemistry. We derive this product form from a spatially resolved kernel representation and establish a perturbative separability criterion that defines the regime where independent variation of the variables is valid. The framework specifies five prescriptor classes for dominant loss pathways in transmon-class devices. Falsifiability is operationalized through a pre-committed 2×2 experimental protocol in which the variables must satisfy independent ratio checks within propagated uncertainty. A Minimum-Dataset Specification standardizes reporting for cross-laboratory inference. Part I establishes the conceptual and mathematical architecture; coordinated experimental validation is reserved for Part II.
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