t b a b a

errors. These new errors introduced on the ancilla may propagate to the code qubits.
This situation must be carefully considered in the algorithm.
6.3 Quantum Code for Correction of Time-Correlated Errors
6.3.1 Outline of the Algorithm
Depending on the aspects mentioned in the last section, the algorithm of quantum error
correction for time-correlated errors is as follows:
1. At the end of an error correction cycle entangle the qubit affected by the error with
two additional ancilla qubits. Thus, if the qubit relapses, the time-correlated error will
propagate to the two additional ancilla qubits.
2. Carry out an extended syndrome measurement of the codeword together with the two
additional ancilla qubits. This syndrome measurement should not alter the state of
the codeword and keep the entanglement between the codeword and the two additional
ancilla qubits intact.
3. Disentangle the two additional ancilla qubits from the codeword and then measure the
two additional ancilla qubits.
4. Carry out the error correction according to the outcomes of Steps 2 and 3.
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