Quantum interference distinguishable photons

Two-photon quantum interference sometimes is ascribed to local interference of the photons at the beamsplitter. However, Kwiat et al. [43] and Pittman et al. [44] demonstrated experimentally that the critical factor is the whether the wave-functions are distinguishable at the detectors. The photons do not need to be at the beamsplitter simultaneously. 

The phenomenon of optical interference is commonly describable in completely classical terms, in which optical fields are represented by classical waves. Classical and quantum theories of optical interference readily explain the presence of an interference pattern, but there are interference effects that distinguish the quantum (photon) nature of light from the wave nature. In this section, we present elementary concepts and definitions of both the classical and quantum theories of optical interference and illustrate the role of optical coherence. 

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