  We present a detailed study on the properties of single photons generated by
spontaneous parametric down conversion (SPDC) when both the spectral and
spatial degrees of freedom are controlled by means of filters. Our results show
that it is possible to obtain pure heralded single photons with high heralding
efficiency despite the use of filters. Moreover, we report an asymmetry on the
single photon properties exhibited in type-II SPDC sources that depends on
choosing the signal or the idler photon as the heralding one.

Quantum Physics;Optics