Second-order surface nonlinear susceptibility in silica nanofibers can be used to obtain a very high-rate source of correlated photon-pairs free of spontaneous Raman scattering. Correlated photon-pair sources are an important resource for quantum communications, as they can be used to generate entangled photon pairs. Generating correlated photon-pairs in silica fibers has been very challenging, typically relying on the four-wave mixing process, a third-order nonlinear processes, which is accompanied by spontaneous Raman scattering that generates unavoidable uncorrelated photons. Second-order nonlinear processes have been used in waveguides. However, coupling light between these waveguides and optical fibers is a source of strong losses. In this article, the authors show that a silica nanofiber can present a strong enough second-order surface nonlinear susceptibility to allow the use of spontaneous parametric down conversion, in this way greatly simplifying the light coupling between the source of correlated photon pairs and a standard single-mode optical fiber.
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