Flip-chip integrated superconducting qubits using electroplated bump bonds

  1. Yen-An Shih,
  2. Rebecca Gharibaan,
  3. Barka Khan,
  4. Dhananjay Joshi,
  5. Siddharth Singh,
  6. Martijn F.S. Zwanenburg,
  7. Eugene Y. Huang,
  8. Nataliia Zhurbina,
  9. Figen Yilmaz,
  10. Lukas Johannes Splitthoff,
  11. Srijit Goswami,
  12. and Christian Kraglund Andersen
Flip-chip integration offers a promising route toward scalable superconducting quantum processors and hybrid semiconductor-superconductor quantum devices. We develop a three-dimensional
transmon architecture using electroplated indium in which the qubit electric field is shared nearly equally between two bump-bonded substrates while maintaining low participation at the indium-bump interface. The resulting geometry is well suited for future hybrid qubits, enabling the integration of distinct material platforms while minimizing sensitivity to bump-interface loss. Using this platform, we evaluate electroplated indium interconnects for superconducting quantum circuits. Flip-chip transmons incorporating electroplated indium bumps exhibit qubit quality factors around 106. In addition, a systematic study of coplanar-waveguide resonators is used to identify losses associated with the electroplating process. In particular, we find that surface losses associated with the gold-layer, used to enable good electric contact with the indium, is likely the primary contributor to the qubit decay rate. These results demonstrate the compatibility of electroplated indium technology with high-coherence superconducting circuits and establish a promising platform for three-dimensional hybrid quantum integration.