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
Resources
- Error budget
The total probability of failure you are willing to accept across a whole computation.
Easy to get wrong: It is divided among every logical qubit and every cycle, so a modest-sounding budget implies a very small per-operation error rate.
Seen in use on /workbench
- Lattice surgery
Performing operations between logical qubits by merging and splitting their patches on the grid.
Easy to get wrong: It needs free space beside the data to move information through, so a register of n logical qubits occupies more than n patches — Microsoft's estimator uses 2n + ⌈√(8n)⌉ + 1.