  Continuous-time quantum error correction (CTQEC) is a technique for
protecting quantum information against decoherence, where both the decoherence
and error correction processes are considered continuous in time. Given any
[[n,k,d]] quantum stabilizer code, we formulate a class of protocols to
implement CTQEC, involving weak coherent measurements and weak unitary
corrections. Under this formalism, we show that the minimal required size of
the ancillary system is n-k+1 qubits, and we propose one scheme that meets this
minimal requirement. Furthermore, we compare our method with other known
schemes, and show that a particular measure of performance described in this
paper is better when using our method.
