• Entanglement and Nonlocality Quantum entanglement was first viewed as a source of paradoxes, most noticeably the EinsteinPodolskyRosen paradox (EPR) , which explicitly suggested that any physical theory must be both localand realistic. Afterward, Bell showed that the correlations among the measurement outputs of space like separated parties on some quantum states cannot be reproduced by a local theory. Bell’s theorem is the first place where the locality assumption is quantified. These conditions then manifested themselves in the socalled Bell inequality, where locality is a crucial assumption but is violated by quantum mechanical predictions. In the Bell’s terminology, quantum mechanics is not a locally causal theory. In what has become standard terminology, this fact is often referred to as quantum nonlocality and has been recognized as the most intriguing quantum feature. Over the past thirty years a very large number of experiments have been conducted with the aim of testing the predictionsof quantum mechanics against those of local hiddenvariable theories. All of them give strong indications against local hidden variable theories. Nonlocality is a fascinating chapter of physics and has attracted much attention since its discovery because it relates two fundamental theories of nature, special relativity and quantum mechanics.
