  Parametric down-conversion (PDC) offers the possibility to control the
fabrication of non-Gaussian states such as Fock states. However, in
conventional PDC sources energy and momentum conservation introduce strict
frequency and photon number correlations, which impact the fidelity of the
prepared state. In our work we optimize the preparation of single-photon Fock
states from the emission of waveguided PDC via spectral filtering. We study the
effect of correlations via photon number resolving detection and quantum
interference. Our measurements show how the reduction of mixedness due to
filtering can be evaluated. Interfering the prepared photon with a coherent
state we establish an experimentally measured fidelity of the produced target
state of 78%.
