  Optical quantum nondemolition devices can provide essential tools for quantum
information processing. Here, we describe several optical interferometers that
signal the presence of a single photon in a particular input state without
destroying it. We discuss both entanglement-assisted and
non-entanglement-assisted interferometers, with ideal and realistic detectors.
We found that existing detectors with 88% quantum efficiency and single-photon
resolution can yield output fidelities of up to 89%, depending on the input
state. Furthermore, we construct expanded protocols to perform QND detections
of single photons that leave the polarization invariant. For detectors with 88%
efficiency we found polarization-preserving output fidelities of up to 98.5%.
