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%.
@article{arxiv.quant-ph/0202046,
title = {Single-photon quantum nondemolition detectors constructed with linear optics and projective measurements},
author = {Pieter Kok and Hwang Lee and Jonathan P. Dowling},
journal= {arXiv preprint arXiv:quant-ph/0202046},
year = {2009}
}