Indium bump bonding
Indium bump bonding is the technique that physically joins two quantum chips face-to-face by squeezing arrays of tiny indium bumps between them. Each bump is a soft metal dot, often only a few tens of micrometers across, deposited on matching pads on the two chips. When the chips are pressed together, the bumps deform and weld into a single column that both holds the chips in place and carries an electrical signal across the gap.
Indium is the metal of choice for a specific reason: it stays soft and ductile even when cooled to a few thousandths of a degree above absolute zero, the temperature at which superconducting qubits operate. Most metals turn brittle in the cold and would crack during the long cooldown, but indium keeps flexing, so the bumps survive. It also becomes superconducting at low temperature, which means a bump can carry a control or readout signal, or a clean ground connection, with very little loss.
In a flip-chip quantum processor this is the concrete bond beneath the architecture. One chip holds the qubits and the other holds the wiring and readout lines; the indium bumps connect the two tiers while leaving a controlled air gap between them. Getting hundreds of bumps to be the same height, land in the right place, and all bond reliably is hard, and bump uniformity, alignment, and yield are active engineering problems on today's small, noisy devices.
A schematic flow: bumps on both chips are aligned and pressed into welded columns that hold the chips and pass signals.
Indium melts at a fairly low temperature and is soft enough to cold-weld under pressure, so many bonds are made without high heat. The same softness that lets the bumps survive cooldown also makes them easy to smear or misalign during assembly, which is part of why yield is a real challenge.