You don't build a road by demolishing neighboring houses: first you lay the asphalt, then you grind away the excess, leaving a smooth surface. That's roughly what physicists in Heidelberg did when they took on the production of Josephson junctions, tiny sandwiches of superconductor and insulator through which current flows without resistance. Previously, such junctions were made by lifting off a protective mask, like demolishing buildings to clear space for a road. That was slow and left ragged edges.

In the new work, they don't demolish the road; they polish it. They deposit a trilayer structure onto a silicon wafer: niobium, aluminum with its oxide, and niobium again. On top, they pour silicon dioxide, like asphalt, and polish it until the top electrode emerges. The polishing pressure is only 138 millibar, about like a breath of wind on your palm, yet it removes 128 nanometers of silicon dioxide per minute. The result is a smooth surface without ragged edges.

The result is impressive: the yield of working junctions exceeded 90% on a wafer. For junctions with a 200-nanometer-thick top electrode, all tested samples were of high quality. The critical current scales linearly with area, and the critical current density was 220±5 A/cm² for one wafer and 549±4 A/cm² for another. Such uniformity is rare in mass production.

Polishing instead of lift-off is like grinding the road rather than demolishing buildings. Previously, to make cross-type junctions, you had to lift off the mask, leaving burrs at the edges that degraded performance. Now, excess silicon dioxide is ground away without touching the junction itself. This speeds up the process and makes it suitable for multilayer integration of superconducting circuits. The method can already be adopted in the production of SQUIDs and superconducting qubits, which require large arrays of uniform junctions.

The road, polished to a shine, leads to quantum computers. The uniformity of junctions directly affects the spread of qubit frequencies, and hence the quality of quantum circuits. The physicists from Heidelberg have shown that such junctions can be made quickly and reliably, without ragged edges. All that remains is to remove the polishing dust, so the road becomes perfect even for the smallest junctions.