A novel single-crystal & single-pass source for polarisation- and   colour-entangled photon pairs
  We demonstrate a new generation mechanism for polarisation- and colour-entangled photon pairs. In our approach we tailor the phase-matching of a periodically poled KTP crystal such that two downconversion processes take place simultaneously. Relying on this effect, our source emits entangled bipartite photon states, emerging intrinsically from a single, unidirectionally pumped crystal with uniform poling period. Its property of being maximally compact and luminous at the same time makes our source unique compared to existing photon-entanglement sources and is therefore of high practical significance in quantum information experiments. 
Fabian Laudenbach, Sebastian Kalista, Michael Hentschel, Philip   Walther, Hannes H\"ubel
quant-ph physics.optics