Al

Al in numbers

Transition temperature

1.2 K

Made as

film

Nitrogen line

Below 77 K

Why engineers choose Al

The reason is oxide. Aluminum grows a thin native oxide that stops at a reproducible thickness, and a Josephson junction is two superconductors separated by a barrier whose thickness sets the behavior of the whole device. Aluminum, its own oxide, and aluminum again gives a junction one fab can make twice. Nothing else on this list offers that, and a processor is hundreds of junctions that all have to agree with each other, so reproducibility beats transition temperature by a wide margin. The rest of the chip can be niobium; the junction is aluminum.

Where Al stops

Its transition temperature is not a temperature, it is a budget item. Everything built on aluminum lives at the bottom of a dilution refrigerator, where the available cooling power is measured in microwatts. That ceiling shapes the architecture above it. How many control lines can enter the cold stage. How much can be dissipated reading a qubit out. And how large a processor gets before the refrigerator, rather than the physics, is the limit.