A simple and efficient quantum non-demolition measurement (QND) scheme is proposed in which the arrival of a signal photon is detected without affecting the qubit state. The proposed QND scheme functions even if the ancillary photon is replaced with weak light composed of vacuum and one-photon states. Although the detection scheme is designed for entanglement sharing applications, it is also suitable for general purification of a single photon state.
@article{arxiv.quant-ph/0702138,
title = {Quantum non-demolition measurement of photon-arrival using an atom-cavity system},
author = {Kunihiro Kojima and Akihisa Tomita},
journal= {arXiv preprint arXiv:quant-ph/0702138},
year = {2007}
}