  Photonics is a promising architecture for the realisation of quantum
information processing, since the two-photon interaction, or non-linearity,
necessary to build logical gates can efficiently be realised by the use of
interference with ancillary photons and detection. Although single-photon
sources and detectors are pivotal in realisations of such systems, clear
guidelines for the required performance of realistic systems are yet to be
defined. We present our detailed numerical simulation of several quantum optics
circuits including sources and detectors all represented in multi-dimensional
Fockspaces, which allows to obtain experimentally realistic performance bounds
for for these devices. In addition, the single-photon source based on switched
parametric down-conversion is studied, which in principle could reach the
required performance. Three approaches for implementing the switching hierarchy
of the photons are simulated, and their anticipated performance is obtained.
Our results define the bar for the optical devices needed to achieve the first
level of linear-optics quantum computing outside the coincidence basis.
