We present a simple but highly efficient source of polarization-entangled photons based on spontaneous parametric down-conversion (SPDC) in bulk periodically poled potassium titanyl phosphate crystals (PPKTP) pumped by a 405 nm laser diode. Utilizing one of the highest available nonlinear coefficients in a non-degenerate, collinear type-0 phase-matching configuration, we generate polarization entanglement via the crossed-crystal scheme and detect 0.64 million photon pair events/s/mW, while maintaining an overlap fidelity with the ideal Bell state of 0.98 at a pump power of 0.025 mW.
@article{arxiv.1204.5330,
title = {A high-brightness source of polarization-entangled photons optimized for applications in free space},
author = {Fabian Steinlechner and Pavel Trojek and Marc Jofre and Henning Weier and Daniel Perez and Thomas Jennewein and Rupert Ursin and John Rarity and Morgan W. Mitchell and Juan P. Torres and Harald Weinfurter and Valerio Pruneri},
journal= {arXiv preprint arXiv:1204.5330},
year = {2012}
}