David K. Kim

Analysis and mitigation of interface losses in trenched superconducting coplanar waveguide resonators

  1. Greg Calusine,
  2. Alexander Melville,
  3. Wayne Woods,
  4. Rabindra Das,
  5. Corey Stull,
  6. Vlad Bolkhovsky,
  7. Danielle Braje,
  8. David Hover,
  9. David K. Kim,
  10. Xhovalin Miloshi,
  11. Danna Rosenberg,
  12. Arjan Sevi,
  13. Jonilyn L. Yoder,
  14. Eric A. Dauler,
  15. and William D. Oliver
Improving the performance of superconducting qubits and resonators generally results from a combination of materials and fabrication process improvements and design modifications that
reduce device sensitivity to residual losses. One instance of this approach is to use trenching into the device substrate in combination with superconductors and dielectrics with low intrinsic losses to improve quality factors and coherence times. Here we demonstrate titanium nitride coplanar waveguide resonators with mean quality factors exceeding two million and controlled trenching reaching 2.2 μm into the silicon substrate. Additionally, we measure sets of resonators with a range of sizes and trench depths and compare these results with finite-element simulations to demonstrate quantitative agreement with a model of interface dielectric loss. We then apply this analysis to determine the extent to which trenching can improve resonator performance.
arxiv pdf

Coherent coupled qubits for quantum annealing

  1. Steven J. Weber,
  2. Gabriel O. Samach,
  3. David Hover,
  4. Simon Gustavsson,
  5. David K. Kim,
  6. Danna Rosenberg,
  7. Adam P. Sears,
  8. Fei Yan,
  9. Jonilyn L. Yoder,
  10. William D. Oliver,
  11. and Andrew J. Kerman
Quantum annealing is an optimization technique which potentially leverages quantum tunneling to enhance computational performance. Existing quantum annealers use superconducting flux
qubits with short coherence times, limited primarily by the use of large persistent currents Ip. Here, we examine an alternative approach, using qubits with smaller Ip and longer coherence times. We demonstrate tunable coupling, a basic building block for quantum annealing, between two flux qubits with small (∼50 nA) persistent currents. Furthermore, we characterize qubit coherence as a function of coupler setting and investigate the effect of flux noise in the coupler loop on qubit coherence. Our results provide insight into the available design space for next-generation quantum annealers with improved coherence.
arxiv pdf