Tailoring the spectral coherence of heralded single photons

Currently, different techniques are known for synthesizing in a tunable fashion the frequency correlations of paired photons generated in spontaneous parametric downconversion (signal-idler correlations). We show that these same techniques can also be used to tune the first-order coherence properties of the signal or idler photons (signal-signal or idler-idler frequency correlations). In consequence, partially coherent radiation can be synthesized through adequate biphoton engineering. Interestingly, when the biphoton is designed to be in a separable state, the signal photons behave as a broadband first-order coherent light source.