Optical quantum computing, quantum information and quantum communication protocols rely on the generation of qubits encoded in optical systems, many of which can be generated via the process of spontaneous parametric downconversion. In this talk, we investigate spectral properties of downconverted states. We characterise the effects of spectral ﬁltering and inefficient detection of a downconverted state used for the heralding of number states. We also introduce a technique for controlling the joint spectral proﬁle of the downconverted photons. By exploiting the dependence of the effective nonlinearity of a periodically poled crystal on its poling order, we tailor the nonlinearity proﬁle and therefore the phasematching function of the downconverted photons. Finally, we consider the validity of the Taylor series expansion of the unitary operator which governs the evolution of the ﬁelds within the crystal, in comparison to the strictly correct time-ordered Dyson series expansion.
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