J. Feigl

Accurate gap control of 3D-integrated superconducting quantum circuits using asymmetric indium architecture and multi-angle flip-chip bonding

  1. L. Richard,
  2. N. Bruckmoser,
  3. J. Feigl,
  4. D. Bunch,
  5. A. Bhattacharjee,
  6. L. Koch,
  7. I. Tsitsilin,
  8. V.P. Bader,
  9. H. Hu,
  10. L. Södergren,
  11. and S. Filipp
Flip-chip 3D integration is a key enabling technology for overcoming the routing and layout constraints of planar superconducting quantum processors, but it requires accurate and uniform
control of the inter-chip spacing while preserving qubit coherence. In conventional processes, thick indium bumps are deposited symmetrically on both chips, while dedicated spacers are introduced to limit the residual tilt. Here, we present a flip-chip bonding process based on an asymmetric indium architecture, combining 20μm thick indium bumps on the control chip with 200nm thin indium pads on the qubit chip. Using electrical and microwave measurements, we demonstrate low resistance and low loss indium interconnects, and show that transmon qubits integrated with this process reach relaxation and coherence times of T1=(159±14)μs and T2,echo=(294±42)μs, respectively, comparable to planar devices. Combined with a multi angle compression procedure that suppresses systematic tilt, this approach reduces the average post bonding tilt to (20±12)μrad. These results establish an asymmetric indium architecture combined with multi angle compression as a robust, coherence preserving route toward 3D integrated superconducting quantum processors.
arxiv pdf

Niobium Air Bridges as a Low-Loss Component for Superconducting Quantum Hardware

  1. N. Bruckmoser,
  2. L. Koch,
  3. I. Tsitsilin,
  4. M.Grammer,
  5. D. Bunch,
  6. L. Richard,
  7. J. Schirk,
  8. F. Wallner,
  9. J. Feigl,
  10. C. M. F. Schneider,
  11. S. Geprägs,
  12. V.P. Bader,
  13. M. Althammer,
  14. L. Södergren,
  15. and S. Filipp
Scaling up superconducting quantum processors requires a high routing density for readout and control lines, relying on low-loss interconnects to maintain design flexibility and device
performance. We propose and demonstrate a universal subtractive fabrication process for air bridges based on an aluminum hard mask and niobium as the superconducting film. Using this technology, we fabricate superconducting CPW resonators incorporating multiple niobium air bridges in and across their center conductors. Through rigorous cleaning methods, we achieve mean internal quality factors in the single-photon limit exceeding Qint=8.2×106. Notably, the loss per air bridge remains below the detection threshold of the resonators. Due to the larger superconducting energy gap of niobium compared to conventional aluminum air bridges, our approach enables stable performance at elevated temperatures and magnetic fields, which we experimentally confirm in temperatures up to 3.9 K and in a magnetic field of up to 1.60 T. Additionally, we utilize air bridges to realize low-loss vacuum-gap capacitors and demonstrate their successful integration into transmon qubits by achieving median qubit lifetimes of T1=51.6μs.
arxiv pdf