Nonlocality and entanglement in a strange system

We show that the relation between nonlocality and entanglement is subtler than one naively expects. In order to do this we consider the neutral kaon system - which is oscillating in time (particle-anti-particle mixing) and decaying - and describe it as an open quantum system. We consider a Bell-CHSH inequality and show a novel violation for non-maximally entangled states. Considering the change of purity and entanglement in time we find that, despite the fact that only two degrees of freedom at a certain time can be measured, the neutral kaon system does not behave like a bipartite qubit system. © Springer-Verlag Berlin Heidelberg 2007.