Beyond Stabilizer Codes II: Clifford Codes
  Knill introduced a generalization of stabilizer codes, in this note called Clifford codes. It remained unclear whether or not Clifford codes can be superior to stabilizer codes. We show that Clifford codes are stabilizer codes provided that the abstract error group has an abelian index group. In particular, if the errors are modelled by tensor products of Pauli matrices, then the associated Clifford codes are necessarily stabilizer codes. 
Andreas Klappenecker (Texas A&M University), Martin Roetteler   (Universitaet Karlsruhe)