<title>Quantum optical experiments and fundamentals of quantum theory</title>

Abstract

Quantum optics has offered new possibilities for experimental tests of basic principles of quantum mechanics. It enables us to experimentally investigate such phenomena as quantum interference and quantum non-locality. By means of cascade transitions or spontaneous parametric down- conversion, entangled photon pairs can be prepared. It makes possible to test the violation of Bell's inequalities and to study other issues connected with the question of completeness of quantum theory. The results of experiments of this kind pronounce in favor of quantum mechanics. The entangled pairs can further be used to demonstrate the violation of other classical inequalities, `non-local' interference in frequency domain, induced interference without induced emission, etc. They are also employed for quantum teleportation. Another family of interesting quantum optical experiments relates to single-photon interference. These experiments demonstrate wave-particle duality and correspondence between the interference visibility and the degree of knowledge of the photon's path in an interferometer. A spectacular example is the so-called interaction-free measurement. Quantum optical experiments open a new window on the quantum world and help us understand it. Further, quantum optics provides the ground for new interesting applications: quantum cryptography, quantum communications, and quantum computation.