We show that coherent error correction with redundant syndrome extraction is
feasible even when realistic error models for the ancilla qubits are used. This
is done by means of a fault-tolerant circuit that has a favorable error
threshold for a conservative error model. It is optimal when Toffoli gates are
available, as they are in many implementations. The possibility of coherent
error correction is important for those quantum computing platforms in which
measurements in the course of the computation are undesirable.Comment: 3 figure