Figure 2

Left: the phase matching function \(|\varphi {|}^{2}\), as a function of ω _p  = ω ₁ + ω ₂, and ω _{_}  = ω ₁ − ω ₂ for collinear Type II. Right: HOM coincidence pattern as a function of the delay Δx = cτ. The count rate is normalised so to be 1 in the absence of interference, for long delays. Both panels refer to the case for a 2-mm β− barium borate (BBO) crystal, phase matched for 775 nm → 1550 nm conversion.