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Publication appeared in Optics Express
Graphical representation of the joint spectral intensity in four different configurations. In the case of coinciding centre wavelengths (λs = λi) Figs. (a), (b) and (d) each represent spectrally indistinguishable signal and idler radiation. However, Figs. (a) and (b) depict cases where signal and idler wavelengths are highly (anti-) correlated, resulting in a purity of P ∼ 0.1. The plots on the bottom each illustrate a frequency-uncorrelated down-conversion (P ∼ 1), where signal and idler carry (c) different and (d) same bandwidth respectively (as indicated by the white curves).