Comment on: "Measuring a Photonic Qubit without Destroying It"
  Recently, Pryde et al reported the demonstration of a quantum non-demolition scheme for single-photon polarization states with linear optics and projective measurements [Phys. Rev. Lett. 92, 190402 (2004)]. Here, we argue that their interpretation of the experiment is inconsistent with the fidelity measure they use. 
Pieter Kok and William J. Munro