Glossary
Every technical term this platform uses, with what it means and — the part that is harder to find elsewhere — the mistake it prevents. On this subject the intuitive reading is often the wrong one, so each entry says what a reader would otherwise get wrong.
Error correction
- Logical qubit
One qubit's worth of protected information, stored across many physical qubits so that errors can be detected and undone.
Easy to get wrong: The count of logical qubits is not the size of the machine. A single logical qubit at code distance 11 occupies 242 physical qubits, and routing space and magic-state factories are on top of that.
Seen in use on /workbench
- Surface code
The most studied way to build a logical qubit, laying physical qubits on a two-dimensional grid where only neighbours interact.
Easy to get wrong: It only suppresses errors below its threshold. Above that, adding more qubits makes the logical error rate worse, not better.
Seen in use on /qec
- Threshold
The physical error rate below which adding more qubits to a code helps, and above which it hurts. Roughly 1% for the surface code.
Easy to get wrong: Results measured above threshold say nothing about behaviour below it. The two are different regimes, not two points on one curve.
Seen in use on /qec/lab
- Leakage
A qubit escaping the two states it is supposed to occupy, into a third level the code cannot describe.
Easy to get wrong: Standard decoders assume it cannot happen, so leakage errors are not merely undetected but outside the model entirely.