A high-brightness source of polarization-entangled photons optimized for   applications in free space
  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. 
Fabian Steinlechner, Pavel Trojek, Marc Jofre, Henning Weier, Daniel   Perez, Thomas Jennewein, Rupert Ursin, John Rarity, M...
quant-ph