Probabilistic quantum non-demolition (QND) measurements can be performed using linear optics and post-selection. Here we show how QND devices of this kind can be used in a straightforward way to implement a quantum relay, which is capable of extending the range of a quantum cryptography system by suppressing the effects of detector noise. Unlike a quantum repeater, a quantum relay system does not require entanglement purification or the ability to store photons.
@article{arxiv.quant-ph/0204097,
title = {Quantum relays and noise suppression using linear optics},
author = {B. C. Jacobs and T. B. Pittman and J. D. Franson},
journal= {arXiv preprint arXiv:quant-ph/0204097},
year = {2009}
}