Alexander P.M. Place

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

Microscopic Relaxation Channels in Materials for Superconducting Qubits

  1. Anjali Premkumar,
  2. Conan Weiland,
  3. Sooyeon Hwang,
  4. Berthold Jäck,
  5. Alexander P.M. Place,
  6. Iradwikanari Waluyo,
  7. Adrian Hunt,
  8. Valentina Bisogni,
  9. Jonathan Pelliciari,
  10. Andi Barbour,
  11. Mike S. Miller,
  12. Paola Russo,
  13. Fernando Camino,
  14. Kim Kisslinger,
  15. Xiao Tong,
  16. Mark S. Hybertsen,
  17. Andrew A. Houck,
  18. and Ignace Jarrige
Despite mounting evidence that materials imperfections are a major obstacle to practical applications of superconducting qubits, connections between microscopic material properties
and qubit coherence are poorly understood. Here, we perform measurements of transmon qubit relaxation times T1 in parallel with spectroscopy and microscopy of the thin polycrystalline niobium films used in qubit fabrication. By comparing results for films deposited using three techniques, we reveal correlations between T1 and grain size, enhanced oxygen diffusion along grain boundaries, and the concentration of suboxides near the surface. Physical mechanisms connect these microscopic properties to residual surface resistance and T1 through losses arising from the grain boundaries and from defects in the suboxides. Further, experiments show that the residual resistance ratio can be used as a figure of merit for qubit lifetime. This comprehensive approach to understanding qubit decoherence charts a pathway for materials-driven improvements of superconducting qubit performance.
arxiv pdf