Ordinary soldering doesn't work for quantum circuits: when cooled to liquid helium temperatures, metal cracks, and the contacts themselves introduce noise. Engineers from the research group tried something different — a drop of gallium alloy that stays liquid at room temperature, like mercury in an old thermometer. Such a bridge fills any irregularities between chips, and when strongly cooled, it itself becomes a superconductor — a conductor with zero resistance.

Tests showed that the signal through the liquid metal is as clean as through the best standard aluminum resonators. Three times, the setup was warmed to room temperature and then cooled again to 15 millikelvins — colder than outer space. After each cycle, the contact worked like new. And when one module was replaced with another, the gallium drop reflowed and restored the superconducting bridge.

The secret to the quality lies in a thin film of tantalum that forms on the surface of the liquid metal. X-rays revealed it's a special crystalline phase that enhances the contact's inductance and helps the signal pass without distortion. At high microwave power, the contact warms up slightly, but this is predictable and doesn't hinder operation.