  We present an experimental demonstration of heralded single photons prepared
in pure quantum states from a parametric downconversion source. It is shown
that, through controlling the modal structure of the photon pair emission, one
can generate pairs in factorable states and thence eliminate the need for
spectral filters in multiple-source interference schemes. Indistinguishable
heralded photons were generated in two independent spectrally engineered
sources, and, by performing a Hong-Ou-Mandel interference between them without
spectral filters at a raw visibility of 94.4%, their purity was measured to be
over 95%.
