Guided tour · Error correction · step 4 of 11
A code never looks at what it protects
Error correction has to detect errors without measuring the data, because measuring it would destroy the superposition.
First, commit to an answer
An X error hits one qubit of a 7-qubit Steane code. How does the decoder find out which qubit?
Decide before you follow the link. Nothing is recorded — the value is in having been wrong on purpose once.
Now go and look
Read the syndrome grid, then try the same error on q2 and q5 and watch the syndrome bits.
What you should see
It never sees the error. It sees six stabilizer bits, and their pattern happens to be the binary index of the qubit that was hit — 001 for q0, 011 for q2, 110 for q5.