Einstein-Podolsky-Rosen paradox and measurement of quantum system

Einstein-Podolsky-Rosen (EPR) paradox is considered in a relation to a measurement of an arbitrary quantum system . It is shown that the EPR paradox always appears in a gedanken experiment with two successively joined measuring devices. PACS number(s): 03.65 Einstein-Podolsky-Rosen paradox is a classical topic in quantum mechanics(for elementary review see [1]). Here we show how the EPR paradox takes place in a process of a measurement of a quantum system by a measuring device, if the process of the measurement is divided on two steps. Let us consider a gedanken setup, consisting of two measuring devices A and B, and an observer, watching the device B. We assume observables to take discrete, nondegenerate values. The gedanken experiment consists of two steps. 1. Device A measures an observable F describing a bypassing elementary particle. Then some (large enough) period of time is waited in order to assure that the particle is at a large distance from the measuring devices. 2. Device B measures the state of the device A and the observer observes the result of this measurement. ∗current address: MS-B258, LANL, 87545 USA