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
- Code distancealso distance, d
The smallest number of physical errors that can corrupt the encoded information without being noticed.
Easy to get wrong: Distance d corrects only (d−1)/2 errors, not d. A distance-3 code corrects one error and *miscorrects* two — the correction itself introduces the failure.
Seen in use on /qec/syndromes
- Stabilizer
A measurement a code makes repeatedly that returns the same answer as long as nothing has gone wrong.
Easy to get wrong: Stabilizers never measure the stored data — doing so would destroy the superposition. They only report whether they were disturbed.
Seen in use on /qec/syndromes
- Syndrome
The pattern of which stabilizers were disturbed: the only information a decoder gets about an error.
Easy to get wrong: An all-zero syndrome does not mean nothing happened. An error heavy enough to commute with every stabilizer is applied silently, which is exactly what a code's distance limits.
Seen in use on /qec/syndromes
- Decoder
The algorithm that reads a syndrome and decides what correction to apply.
Easy to get wrong: A decoder succeeds when its guess differs from the real error by a stabilizer, not when it guesses correctly. Scoring by exact match reports failures on codes that in fact corrected everything.
Seen in use on /decoders
- MWPMalso minimum-weight perfect matching, matching
A decoder that pairs up detection events so the total 'distance' between paired events is as small as possible.
Easy to get wrong: Fast and accurate on surface codes, but its accuracy depends on the noise matching the model it was given. A decoder tuned for depolarizing noise can do poorly when readout error dominates.
Seen in use on /qec/lab
- 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
- Logical error rate
How often the protected information is corrupted despite error correction, per round or per computation.
Easy to get wrong: Zero observed failures is not a rate of zero. Zero failures in 10,000 shots bounds the rate below roughly 3.8×10⁻⁴ and says nothing about whether it is 10⁻⁵ or 3×10⁻⁴.
Seen in use on /leaderboard
- Physical error rate
How often an individual hardware operation goes wrong.
Easy to get wrong: It is an assumption in a simulation, not a measurement of your hardware, unless the record says otherwise.
Seen in use on /qec/lab
- Noise model
The description of how a device is assumed to fail, used to simulate it.
Easy to get wrong: Two decoders compared under different noise models are not being compared. Depolarizing, readout and leakage noise stress a decoder in different ways.
Seen in use on /qec/lab
- Depolarizing noisealso depolarizing
The simplest useful noise model: with some probability, a random Pauli error replaces the intended state.
Easy to get wrong: It is the friendliest realistic model. A decoder that only works under depolarizing noise has not been shown to work.
- 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.