Superconducting Qubits

A superconductor is a material that carries electricity with exactly zero resistance once it is cold enough. A superconducting qubit is a small circuit made of such material. At heart it is a capacitor (a part that stores charge) joined to a Josephson junction. The junction is a very thin wall of insulator, nanometres thick, between two superconductors. Current can still sneak through it by a quantum effect called tunnelling.

The junction is the key part. At this tiny, cold scale, the circuit's energy cannot take just any value. It can only sit at certain set amounts, called energy levels, like rungs on a ladder. A normal circuit of this kind has evenly spaced rungs. Then a signal that lifts it from rung 0 to rung 1 also lifts it from rung 1 to rung 2. You could not aim at one step alone. The junction makes the spacing uneven, like the energy levels of an atom. Now a control pulse tuned to the gap between the two lowest rungs moves only that step. Those two rungs act as 0 and 1.

So the qubit is an artificial atom, printed on a chip. The most common design is called a transmon. It is built so that stray electric charge nearby barely disturbs it.

Being printed is both the strength and the weakness. Chip-printing methods can make hundreds of qubits on one chip. But unlike real atoms, no two come out exactly the same. It is like baking cookies: every cookie comes from one recipe, yet each one bakes a little differently. So each qubit must be measured, and its control pulses tuned to match. This constant tuning is called calibration. The chip's performance drifts between calibrations. That is why published error rates carry dates.