Adapted from S. Massar and S. Pironio, Physics 5, 56 (2012)

Figure 1: Entangled quantum states that are local can be combined in a global quantum state that is nonlocal. (a) A source $S$ distributes two parts of an entangled state $ρ$ to two observers. The state $ρ$ is prepared to be local: the correlation statistics measured by the observers can never be distinguished from the classical case as they do not violate any Bell’s inequality. The state has zero nonlocality. (b) A nonlocal state can be constructed by combining several copies of $ρ$. The two observers perform simultaneous measurements on all copies of $ρ$. Palazuelos shows that the observed correlation statistics for the combined state can violate a Bell’s inequality, i.e., the state is nonlocal. Loosely speaking, zero plus zero can thus be greater than zero in quantum mechanics.