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.

Thermal resistance in superconducting flip-chip assemblies

  1. Joel Hätinen,
  2. Emma Mykkänen,
  3. Klaara Viisanen,
  4. Alberto Ronzani,
  5. Antti Kemppinen,
  6. Lassi Lehtisyrjä,
  7. Janne S. Lehtinen,
  8. and Mika Prunnila
Cryogenic microsystems that utilize different 3D integration techniques are being actively developed, e.g., for the needs of quantum technologies. 3D integration can introduce opportunities
and challenges to the thermal management of low temperature devices. In this work, we investigate sub-1 K inter-chip thermal resistance of a flip-chip bonded assembly, where two silicon chips are interconnected by indium bumps by atmospheric thermocompression bonding. The temperature dependence of the inter-chip thermal resistance follows the power law of αT−3, with α=7.7−15.4 K4 μm2/nW and a thermal contact area of 0.306 mm2. The T−3 relation indicates phononic interfacial thermal resistance, which is supported by the vanishing electrical thermal conduction due to the superconducting interconnections. Such a thermal resistance value can introduce a thermalization bottleneck, which can be detrimental for some applications, but it can also be harnessed. We provide a study of the latter case by simulating the performance of solid-state junction microrefrigerator where we use the measured thermal resistance value.

Characterizing Low-Quality-Factor Dissipative Superconducting Resonators

  1. Yu-Cheng Chang,
  2. Bayan Karimi,
  3. Jorden Senior,
  4. Alberto Ronzani,
  5. Joonas T. Peltonen,
  6. Hsi-Sheng Goan,
  7. Chii-Dong Chen,
  8. and Jukka P. Pekola
Characterizing superconducting microwave resonators with highly dissipative elements is a technical challenge, but a requirement for implementing and understanding the operation of
hybrid quantum devices involving dissipative elements, e.g. for thermal engineering and detection. We present experiments on λ/4 superconducting niobium coplanar waveguide (CPW) resonators, shunted at the antinode by a dissipative copper microstrip via aluminium leads, yielding a quality factor unresolvable from the typical microwave environment. By measuring the transmission both above and below this transition, we are able to isolate the resonance. We then experimentally verify this method with copper microstrips of increasing thicknesses, from 50 nm to 150 nm, and measure quality factors in the range of 10∼67 in a consistent way.