Quantum optics: quantum, classical, and metaphysical aspects

Presented below is the critical review of the modern optical experiments devoted to the demonstration of the quantum nature of light and which reveal the properties of photons. The three main languages used for the description of such demonstration experiments are described and compared: the formal quantum language Q, which enables calculation of all averaged experimental data; the classical or semiclassical language C, which allows a visual qualitative description of some effects; the metaphysical language M, which uses vaguely defined terms (such as photons, their duality, quantum nonlocality, etc.) and provides no new observable results, but claims to offer the most profound reflection of the quantum-optical phenomena. It is proposed to distinguish the three types of photons: Q-photon (the Fock state with n=1), C-photon (the classical wave packet), and M-photon (the hypothetical elementary particle producing discrete pulses at the photon detector output, and which has not yet been defined in the framework of any consistent theory).