Quantum Non-demolition Measurements on Qubits
  We discuss the characterization and properties of quantum non-demolition (QND) measurements on qubit systems. We introduce figures of merit which can be applied to systems of any Hilbert space dimension thus providing universal criteria for characterizing QND measurements. We discuss the controlled-NOT gate and an optical implementation as examples of QND devices for qubits. We also discuss the QND measurement of weak values. 
T.C.Ralph, S.D.Bartlett, J.L.O'Brien, G.J.Pryde and H.M.Wiseman