The biggest advantage of quantum technology is probably the fact that it has access to quantum entanglement. Therefore, reliable and copious sources of entangled photons are a crucial ingredient of the photonic hardware of the future. In this work, Yan-Xiao Gong and colleagues have tailored a source of polarization entangled photons with the astounding brightness of over 10 million photon pairs per second (per mW of pump power, per nm of spectral bandwidth). To achieve high brightness, we need a strong coherent pump to spend enough time inside a nonlinear medium, but long nonlinear crystals are usually riddled with problems due to the constraints that are imposed by the geometry of the experiment. To get around these difficulties, we typically have to accept working with shorter crystals. The way Gong and colleagues got around these issues was by optimizing the design of a periodically poled nonlinear waveguide, which has been carefully grown according to the specs that they calculated, down to the temperature of the medium. The result is an extremely bright source that has the advantage of allowing to separate the pairs deterministically. To prove the quality of the entanglement, the researchers have violated Bell's inequalities by 25 standard deviations.
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