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In 1935, the Einstein-Podolsky-Rosen paper introduced a new kind of two-particles quantum state, which was named "entangled" by Schrödinger. Showing that measurements on both particles were strongly correlated even if the particles were widely separated, Einstein argued that the formalism did not describe all the elements of the physical reality, and he concluded that Quantum Mechanics was not complete. Bohr immediately opposed this reasoning, claiming that Quantum Mechanics was consistent as it was, and that it did not need to be completed, and even that it should not be completed.The discussion between Einstein and Bohr lasted until the death of the two giants, leaving the question unsettled. It was considered by most of the physicists only as an epistemological debate, holding on questions of interpretation, but without any practical consequence on how to use Quantum Mechanics. The situation changed in 1964, when Bell discovered that taking Einstein's point of view led to consequences in contradiction with the predictions Quantum Mechanics in some (rare) situations. The debate was then displaced from the realm of epistemology to the one of physics, since it could be settled by experiments.After a series of experiments closer and closer to the ideal Gedankenexperiment, more and more physicists realized that entanglement was definitely weirder than any previous concept, and that it might be used for new ways of processing and transmitting information. This prompted the development of a new field, quantum information, which brings us into a new quantum age, and may change our society as dramatically as the first quantum revolution, which gave us integrated circuits and lasers, responsible for the development of the information and communication society.

DOI: 10.1007/978-3-319-14316-3_6

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