We show how to create practical, efficient, quantum repeaters, employing double-photon guns, for long-distance optical quantum communication. The guns create polarization-entangled photon pairs on demand. One such source might be a semiconducter quantum dot, which has the distinct advantage over parametric down-conversion that the probability of creating a photon pair is close to one, while the probability of creating multiple pairs vanishes. The swapping and purifying components are implemented by polarizing beam splitters and probabilistic optical CNOT gates.
@article{arxiv.quant-ph/0203134,
title = {Practical quantum repeaters with linear optics and double-photon guns},
author = {Pieter Kok and Colin P. Williams and Jonathan P. Dowling},
journal= {arXiv preprint arXiv:quant-ph/0203134},
year = {2009}
}