Native-oxide-passivated trilayer junctions for superconducting qubits

  1. Pankaj Sethi,
  2. Om Prakash,
  3. Jukka-Pekka Kaikkonen,
  4. Mikael Kervinen,
  5. Elsa T. Mannila,
  6. Mário Ribeiro,
  7. Debopam Datta,
  8. Christopher W. Förbom,
  9. Jorden Senior,
  10. Renan P. Loreto,
  11. Joel Hätinen,
  12. Klaara Viisanen,
  13. Jukka I. Väyrynen,
  14. Alberto Ronzani,
  15. Antti Kemppinen,
  16. Visa Vesterinen,
  17. Mika Prunnila,
  18. and Joonas Govenius
Superconducting qubits in today’s quantum processing units are typically fabricated with angle-evaporated aluminum–aluminum-oxide–aluminum Josephson junctions. However,
there is an urgent need to overcome the limited reproducibility of this approach when scaling up the number of qubits and junctions. Fabrication methods based on subtractive patterning of superconductor–insulator–superconductor trilayers, used for more classical large-scale Josephson junction circuits, could provide the solution but they in turn often suffer from lossy dielectrics incompatible with high qubit coherence. In this work, we utilize native aluminum oxide as a sidewall passivation layer for junctions based on aluminum–aluminum-oxide–niobium trilayers, and use such junctions in qubits. We design the fabrication process such that the few-nanometer-thin native oxide is not exposed to oxide removal steps that could increase its defect density or hinder its ability to prevent shorting between the leads of the junction. With these junctions, we design and fabricate transmon-like qubits and measure time-averaged coherence times up to 30 μs at a qubit frequency of 5 GHz, corresponding to a qubit quality factor of one million. Our process uses subtractive patterning and optical lithography on wafer scale, enabling high throughput in patterning. This approach provides a scalable path toward fabrication of superconducting qubits on industry-standard platforms.

Methods to achieve near-millisecond energy relaxation and dephasing times for a superconducting transmon qubit

  1. Mikko Tuokkola,
  2. Yoshiki Sunada,
  3. Heidi Kivijärvi,
  4. Leif Grönberg,
  5. Jukka-Pekka Kaikkonen,
  6. Visa Vesterinen,
  7. Joonas Govenius,
  8. and Mikko Möttönen
Superconducting qubits are one of the most promising physical systems for implementing a quantum computer. However, executing quantum algorithms of practical computational advantage
requires further improvements in the fidelities of qubit operations, which are currently limited by the energy relaxation and dephasing times of the qubits. Here, we report our measurement results of a high-coherence transmon qubit with energy relaxation and echo dephasing times surpassing those in the existing literature. We measure a qubit frequency of 2.890 GHz, an energy relaxation time with a median of 502 us and a maximum of (765 +/- 82.6) us, and an echo dephasing time with a median of 541 us and a maximum of (1057 +/- 138) us. We report details of our design, fabrication process, and measurement setup to facilitate the reproduction and wide adoption of high-coherence transmon qubits in the academia and industry.