Hi, What crystal do I need to generate polarization entangled photons? I read BBO (beta-BaB2O4) crystal are needed but when I call the engineers at commercial companies and mention the term "entangled photons" it goes way over their heads and they just say they don't make this crystal. I am trying to use a laser diode at 856nm (give or take) and send it trough a second harmonic generator (now at 428nm) to hit the BBO crystal with. Much like it is described in the paper : http://xqp.physik.uni-muenchen.de/publ/apl_79_869_2001.pdf Anyone has any suggestions - for a good company or share their experience with this field? Thanks in advance!