Single-photon quantum nondemolition detectors constructed with linear optics and projective measurements
Physical Review A 66, 063814 (2002)
Optical quantum nondemolition devices can provide an essential tool 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 using (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. These interferometers are not dependent on non-detection, contrary to the previous one, and as a consequence, the fidelity of the output state is much higher. For detectors with 88% efficiency we found polarizations preserving output fidelities of up to 98.5%. Finally, the fidelities our interferometers provide a dramatic example of why we should not condition the output on non-detection events.