• Entanglement and Non-locality

Quantum entanglement was first viewed as a source of paradoxes, most noticeably the Einstein-Podolsky-Rosen 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 so-called 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 non-locality 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 hidden-variable theories. All of them give strong indications against  local hidden variable theories. Non-locality 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.