Quantum entanglement -- Review of problems and solutions.
All puzzling emergences of quantum mechanics arise from the special feature, innate in the quantum formalism, known as entanglement. This characteristic of quantum mechanics is due to the peculiar relation between the space of states of a compound system and the spaces of states of its components. In ordinary circumstances entanglement causes no conceptual problem, but there are two kinds of situation in which worries are justified. In the first case, where entanglement involves different states of far apart particles, the problem was raised by Einstein, Podolsky and Rosen (EPR). In the second one, where the entangled state contains distinguishable conditions of a macroscopic object, the problem arises in the instance of measurement and was stressed by Schrödinger. The stories of discussions that took place with reference to the two problems will be reviewed. It will be seen that generally agreed conclusions have been reached in the EPR case. On the other hand, the debate is still alive as regards measurement, the proposals still on the table ranging from denying the existence of any problem to adding elements to the standard concept of quantum state, or to altering the evolution law given by the Schrödinger equation.