  It is shown, theoretically and experimentally, that at any type-II
spontaneous parametric down-conversion (SPDC) phase matching, the
decoherence-free singlet Bell state is always present within the natural
bandwidth and can be filtered out by a proper spectral selection. Instead of
the frequency selection, one can perform time selection of the two-photon time
amplitude at the output of a dispersive fibre. Applications to quantum
communication are outlined.
