Hidden quantum nonlocality revealed by local filters

A family of two spin 1 2 states with the following properties is presented. These states are “local” in the sense that they do not violate any Bell—CHSH inequality. However, after each spin interacts with two independent local environments, the resulting states of the two spin 1 2 systems violate a Bell inequality. It is argued that: (1) The problem of classifying the nonlocal states of two spin 1 2 systems is still open. In particular, for mixed states the violation of Bell's inequality and the concept of nonlocality differ, (2) The fact that some dissipative environments can increase quantum correlations might be useful for quantum computation. (3) Careless application of generalized quantum measurements can violate Bell's inequality by more than 2√2, even for mixtures of product states.