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
- 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
- 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.