ИСТИНА

Fock-states of an order higher than two are desirable objects for quantum technologies due to their nonclassical correlation properties. Particularly, they are of interest for production of Greenberg-HorneZeilinger states [1] and optical quantum computing [2]. For three-photon states several attempts have been done already, based on seeding quantum fluctuations in one or two desirable modes [3,4], cascading two-photon parametric down-conversion [5] or special optical fibers, allowing one to achieve required spatial modes of pump and scattered photons [6,7]. Our work presents theoretical investigation of third order parametric down-conversion in birefringent crystals, namely we focus on those with inversion centers, having zero and hence prohibiting all threewave processes. The analysis includes the calculation of differential count rate in unit frequency and angular range, spectral distribution of correlated photons for type I and type II phasematching, total count rate considering limited detection ranges and measurement time, required for distinguishing signal coincidences from noise ones. The comparison with triplets, generated in optical fibers, is also presented.