QUANTUM ENTANGLEMENT - CRITERIA AND COLLECTIVE TESTS

The state of a quantum system, consisting of two distinct subsystems, is called separable if it can be prepared by two distant experimenters who receive instructions from a common source, via classical communic ation channels. A necessary condition is derived and is shown to be mo re sensitive than Bell's inequality for detecting quantum inseparabili ty. Moreover, collective tests of Bell's inequality (namely, tests tha t involve several composite systems simultaneously) may sometimes lead to a violation of Bell's inequality, even if the latter is satisfied when each composite system is tested separately.