Dolan-bridge (shadow-evaporated) junction

The Dolan-bridge junction is the classic, almost handmade way of building a Josephson junction: the tiny sandwich of aluminum, a few-atom-thick layer of aluminum oxide, and aluminum again that sits at the heart of most superconducting qubits. The clever part is that the whole stack is laid down in a single chip-making run, without ever opening the vacuum chamber, using a trick of geometry and shadows. It matters because almost every superconducting quantum chip made in a university lab today is built this way.

First, electron-beam lithography carves a stencil in resist with a thin free-standing bridge of resist suspended over a gap, like a tiny footbridge. Aluminum is then evaporated straight down at one tilt angle; the bridge casts a shadow, so the metal lands on one side of the gap. The chip is exposed to a whiff of oxygen so a thin oxide skin grows on that first aluminum. Then aluminum is evaporated again at the opposite tilt; now the shadow falls the other way, and the second layer lands shifted over, partly overlapping the first. Where the two layers cross, with the oxide in between, you get an Al/AlOx/Al overlap, and that overlap is the junction. The same suspended bridge that makes the shadow gives the method its name.

It is simple and cheap, but it is also variable. The junction's electrical strength depends on the overlap area and the oxide thickness, both set by angles, timing, and a self-grown oxide that is hard to control to the atom. So nominally identical junctions come out a few percent apart, and since that strength sets a qubit's frequency, the junctions drift apart in frequency too. On chips with many qubits this frequency spread causes collisions, which is why labs are working on tighter recipes and, for large-scale production, alternative junction styles.