Indium Electroplating for Scalable Integration of Superconducting Qubits

Quantum computing to be fault tolerant it is necessary to reach the number of qubits to at least thousands. To this end, it is important to develop low-loss interconnects between the different components of the quantum processor. We report high qubit coherence made with flip-chip bonded electroplated indium bumps. The employed metal stack, bump and seed layer, performs electrically as good as the thermally-evaporated indium bumps in cryogenic environment [1]. In addition, the fabrication process is compatible with Josephson junction manufacturing keeping the average coherence time T1 ∼ 50µs. Electroplating technology allows growing bumps as tall as 20 µm which reduces crosstalk and increases freedom in design choices allowing for example stronger coupling between elements by reducing stray coupling to the opposite chiplet.

[1] T. Brecht et. al., Appl. Phys. Lett. 107, 192603 (2015)