Three-photon frequency down-conversion as an event-ready source of entangled states

Abstract

An event-ready (or deterministic) source of entangled states is of interest in the practical implementation of schemes of quantum computation and cryptography as well as in the experiments aimed to elucidate the foundations of quantum mechanics. I study the phase matching geometry and detection conditions for the process of three-photon frequency down-conversion, which is a possible source of that kind. It is apparently free of some limitations of the schemes based on two-photon frequency down-conversion. The goal of the proposed setup is not the generation of an entangled state of three photons (in a GHZ-state sense), but of an entangled state of two photons with a "trigger" signal. As an example, I estimate the values of the trios' flux and trigger's reliability for the particular case of crystal calcite illuminated by the fourth harmonic of a Nd:YAG mode-locked laser.