Curator's Take
AI Commentary
This article demonstrates that electroplated indium bump bonds can be used to build flip‑chip transmons with quality factors near 10⁶, showing that a fully three‑dimensional architecture is compatible with the coherence levels required for error‑corrected processors. By sharing the qubit electric field almost equally between two bonded substrates while keeping loss at the indium interface low, the work opens a practical path toward heterogeneous integration of superconducting and semiconductor quantum devices. The systematic identification of gold‑layer surface loss also gives the community a clear target for further material optimisation as modular, multi‑chip scaling becomes a priority.
— Mark Eatherly
Summary
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 $10^6$. 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.