Yizhou Huang

Identification and Mitigation of Conducting Package Losses for Quantum Superconducting Devices

  1. Yizhou Huang,
  2. Yi-Hsiang Huang,
  3. Haozhi Wang,
  4. Zach Steffen,
  5. Jonathan Cripe,
  6. F. C. Wellstood,
  7. and B. S. Palmer
Low-loss superconducting microwave devices are required for quantum computation. Here, we present a series of measurements and simulations showing that conducting losses in the packaging
of our superconducting resonator devices affect the maximum achievable internal quality factors (Qi) for a series of thin-film Al quarter-wave resonators with fundamental resonant frequencies varying between 4.9 and 5.8 GHz. By utilizing resonators with different widths and gaps, we sampled different electromagnetic energy volumes for the resonators affecting Qi. When the backside of the sapphire substrate of the resonator device is adhered to a Cu package with a conducting silver glue, a monotonic decrease in the maximum achievable Qi is found as the electromagnetic sampling volume is increased. This is a result of induced currents in large surface resistance regions and dissipation underneath the substrate. By placing a hole underneath the substrate and using superconducting material for the package, we decrease the ohmic losses and increase the maximum Qi for the larger size resonators.
arxiv pdf

Characterization of Asymmetric Gap-Engineered Josephson Junctions and 3D Transmon Qubits

  1. Zach Steffen,
  2. S. K. Dutta,
  3. Haozhi Wang,
  4. Kungang Li,
  5. Yizhou Huang,
  6. Yi-Hsiang Huang,
  7. Advait Mathur,
  8. F. C. Wellstood,
  9. and B. S. Palmer
We have fabricated and characterized asymmetric gap-engineered junctions and transmon devices. To create Josephson junctions with asymmetric gaps, Ti was used to proximitize and lower
the superconducting gap of the Al counter-electrode. DC IV measurements of these small, proximitized Josephson junctions show a reduced gap and larger excess current for voltage biases below the superconducting gap when compared to standard Al/AlOx/Al junctions. The energy relaxation time constant for an Al/AlOx/Al/Ti 3D transmon was T1 = 1 {\mu}s, over two orders of magnitude shorter than the measured T1 = 134 {\mu}s of a standard Al/AlOx/Al 3D transmon. Intentionally adding disorder between the Al and Ti layers reduces the proximity effect and subgap current while increasing the relaxation time to T1 = 32 {\mu}s.
arxiv pdf