We have to resort to increasingly smaller specks of matter to store, process, and transport information because we have to reduce the physical dimensions of our devices and their power dissipation. At the same time we have to develop increasingly more powerful computers and more secure and reliable communication systems. Thus, sooner or later we have to build computers and communication systems based upon quantum effects and think about verification of their properties. Quantum information has special properties: the state of a quantum system cannot be measured or copied without disturbing it; quantum state can be entangled, two systems have a definite state though neither has a state of its own; superposition - we cannot reliably distinguish non-orthogonal states of a quantum system. In this paper we discuss the concepts of quantum states and measurements, present Bell states, and Bell's inequality.
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