Figure 1
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.