Bad Qubits — Learn Quantum by Doing

Multiple choice

The lesson stresses that a surface-code decoder does not act on the raw value of a stabilizer check, but on a 'detection event'. How is the detection event for a check defined across rounds?

detector = s_t (+) s_(t-1), the XOR of the check's outcome in successive rounds
detector = s_t + s_(t-1) + s_(t-2), the sum over three rounds
detector = s_t AND s_(t-1), the logical AND of two consecutive rounds
detector = s_t, just the current round's raw value