A bridge of aluminum just a few atoms thick is the heart of many quantum devices. Current flows through such a bridge without resistance, and it can be controlled with a magnetic field. These bridges are called Josephson junctions, after the British physicist who described their behavior. They are usually made with an electron beam, but that is expensive and slow.

The new method does away with the electron beam. First, a mask is patterned on the substrate using light (photolithography), then excess metal is removed with a stream of argon ions, like sandblasting, but on a microscopic scale. The resulting junctions have areas from 1 to 6 square micrometers — roughly the size of a few bacteria laid end to end. By varying the oxidation conditions, the junction's resistance can be tuned by a factor of 100.

Junctions made this way were assembled into a superconducting quantum interference device (SQUID)—an instrument that senses magnetic fields with incredible precision. SQUIDs made with the new method showed less chip-to-chip variation than those made with electron-beam lithography. And when a SQUID was embedded in a cavity and used as a parametric amplifier, it boosted weak signals by 40 decibels, like turning a whisper into the roar of a rock concert.

This simple fabrication method for Josephson junctions could become the basis for mass-producing superconducting quantum circuits. Less complexity means lower cost, bringing quantum computers and sensitive detectors closer to everyday life.