Stephen A. Lyon

Superconducting Gap Engineering in Tantalum-Alloy-Based Resonators

  1. Chen Yang,
  2. Faranak Bahrami,
  3. Guangming Cheng,
  4. Mayer Feldman,
  5. Nana Shumiya,
  6. Stephen A. Lyon,
  7. Nan Yao,
  8. Andrew A. Houck,
  9. Nathalie P. de Leon,
  10. and Robert J. Cava
Utilizing tantalum (Ta) in superconducting circuits has led to significant improvements, such as high qubit lifetimes and quality factors in both qubits and resonators, underscoring
the importance of material optimization in quantum device performance. In this work, we explore superconducting gap engineering in Ta-based devices as a strategy to expand the range of viable host materials. By alloying 20 atomic percent hafnium (Hf) into Ta thin films, we achieve a superconducting transition temperature (Tc) of 6.09~K, as measured by DC transport, reflecting an increased superconducting gap. We systematically vary deposition conditions to control film orientation and transport properties of the Ta-Hf alloy films. The enhancement in Tc is further confirmed by microwave measurements at millikelvin temperatures. Despite the 40\% increase in Tc relative to pure Ta, the loss contributions from two-level systems (TLS) and quasiparticles (QPs) remain unchanged in the low-temperature regime. These findings highlight the potential of material engineering to improve superconducting circuit performance and motivate further exploration of engineered alloys for quantum technologies.
arxiv pdf

Disentangling Losses in Tantalum Superconducting Circuits

  1. Kevin D. Crowley,
  2. Russell A. McLellan,
  3. Aveek Dutta,
  4. Nana Shumiya,
  5. Alexander P.M. Place,
  6. Xuan Hoang Le,
  7. Youqi Gang,
  8. Trisha Madhavan,
  9. Nishaad Khedkar,
  10. Yiming Cady Feng,
  11. Esha A. Umbarkar,
  12. Xin Gui,
  13. Lila V. H. Rodgers,
  14. Yichen Jia,
  15. Mayer M. Feldman,
  16. Stephen A. Lyon,
  17. Mingzhao Liu,
  18. Robert J. Cava,
  19. Andrew A. Houck,
  20. and Nathalie P. de Leon
Superconducting qubits are a leading system for realizing large scale quantum processors, but overall gate fidelities suffer from coherence times limited by microwave dielectric loss.
Recently discovered tantalum-based qubits exhibit record lifetimes exceeding 0.3 ms. Here we perform systematic, detailed measurements of superconducting tantalum resonators in order to disentangle sources of loss that limit state-of-the-art tantalum devices. By studying the dependence of loss on temperature, microwave photon number, and device geometry, we quantify materials-related losses and observe that the losses are dominated by several types of saturable two level systems (TLSs), with evidence that both surface and bulk related TLSs contribute to loss. Moreover, we show that surface TLSs can be altered with chemical processing. With four different surface conditions, we quantitatively extract the linear absorption associated with different surface TLS sources. Finally, we quantify the impact of the chemical processing at single photon powers, the relevant conditions for qubit device performance. In this regime we measure resonators with internal quality factors ranging from 5 to 15 x 10^6, comparable to the best qubits reported. In these devices the surface and bulk TLS contributions to loss are comparable, showing that systematic improvements in materials on both fronts will be necessary to improve qubit coherence further.
arxiv pdf