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Abstract

A method and device for generating a choice of individual photons or photon pairs in an optical channel. The generation of this choice involves generating a two-photon state corresponding to a photon pair and spatially separating the photon pair while preserving the quantum-mechanical correlation, in the event the photons are emitted in a collinear manner. One photon is coupled into one optical channel each, the one channel containing an interferometer with variable optical path-length difference .delta.1.sub.F -.delta.1.sub.S, and the other channel including an optical delay section having the optical length .delta.1. The channels are spatially reunited through a beam splitter, and an adjustment of the quantities .delta.1.sub.F, .delta.1.sub.S, and .delta.1 occurs in such a way that the probability K for coincidences between the outputs of the beam splitter is a choice of approximately K=0 or approximately K=1, or approximately a predetermined intermediate value, with K=0 corresponding to a photon pair in one of the output channels of the beam splitter, and K=1 corresponding to two individual photons in both output channels of the beam splitter. With this choice generation, an optical separating filter or gate can be implemented for a choice of one-photon or two-photon states, which is usable in quantum cryptography and as a basic element of a quantum-optical computing machine.

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