Differential indium bonding for high-coherence 3D-integrated superconducting qubits

To address classically intractable problems, superconducting quantum processors must scale to larger system sizes. However, routing lines to an increasing number of qubits remains a challenge. Flip-chip bonding enables higher connectivity by separating qubit and control elements onto two chips. This technique relies on thick indium bumps located on both chips to ensure proper mechanical and electrical connection. However, the additional fabrication steps may introduce new loss channels and degrade performance. Here, we demonstrate high coherence qubits using a differential indium bonding structure, concentrating most of the indium on the control chip. This approach minimizes processing on the qubit chip and reduces the risk of contamination of the Josephson junctions. We also show the compatibility of this process with precise gap targeting using polymer spacers, achieving a standard deviation of 70 nm and a tilt of 15 µrad on average.