Merged-Element Transmons: Design and Qubit Performance

  1. H. J. Mamin,
  2. E. Huang,
  3. S. Carnevale,
  4. C. T. Rettner,
  5. N. Arellano,
  6. M. H. Sherwood,
  7. C. Kurter,
  8. B. Trimm,
  9. M. Sandberg,
  10. R. M. Shelby,
  11. M. A. Mueed,
  12. B. A. Madon,
  13. A. Pushp,
  14. M. Steffen,
  15. and D. Rugar
We have demonstrated a novel type of superconducting transmon qubit in which a Josephson junction has been engineered to act as its own parallel shunt capacitor. This merged-element transmon (MET) potentially offers a smaller footprint and simpler fabrication than conventional transmons. Because it concentrates the electromagnetic energy inside the junction, it reduces relative electric field participation from other interfaces. By combining micrometer-scale Al/AlOx/Al junctions with long oxidations and novel processing, we have produced functional devices with EJ/EC in the low transmon regime (EJ/EC ≲30). Cryogenic I-V measurements show sharp dI/dV structure with low sub-gap conduction. Qubit spectroscopy of tunable versions show a small number of avoided level crossings, suggesting the presence of two-level systems (TLS). We have observed mean T1 times typically in the range of 10-90 microseconds, with some annealed devices exhibiting T1 > 100 microseconds over several hours. The results suggest that energy relaxation in conventional, small-junction transmons is not limited by junction loss.
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