  The error correcting capabilities of the Calderbank-Shor-Steane [[7,1,3]]
quantum code, together with a fault-tolerant syndrome extraction by means of
several ancilla states, have been numerically studied. A simple probability
expression to characterize the code ability for correcting an encoded qubit has
been considered. This probability, as a correction quality criterion, permits
the error correction capabilities among different recovery schemes to be
compared. The memory error threshold is calculated by means of the best method
of those considered.
