We describe a scheme that allows for the generation of any desired N-photon
state on demand. Under ideal conditions, this requires only N single photon
sources, laser pulses and linear optics elements. First, the sources should be
initialised with the help of single-qubit rotations and repeat-until-success
two-qubit quantum gates [Lim et al., Phys. Rev. Lett. 95, 030305 (2005)].
Afterwards, the state of the sources can be mapped onto the state of N newly
generated photons whenever needed.Comment: 9 pages, 3 figure