When I read about entanglement it is described as the singlet state two particles (or eventually even molecules) are in after interaction with each others or due to a decay. Put in that way it does not sound difficult to do. Just let two particles scatter with each others and we should have an entangled system. But when I read about the practical realizations we get to know about entanglement as something tremendously difficult to realize in laboratory. So, I'm probably missing something here.... Can someone help?