  The third-order non-linearity of silicon gives rise to a spontaneous
four-wave mixing process in which correlated photon pairs are generated.
Sources based on this effect can be used for quantum computation and
cryptography, and can in principle be integrated with standard CMOS fabrication
technology and components. However, one of the major challenges is the on-chip
demultiplexing of the photons, and in particular the filtering of the pump
power, which is many orders of magnitude larger than that of the signal and
idler photons. Here, we propose a photonic crystal coupled-cavity system
designed so that the coupling of the pump mode to the output channel is
strictly zero due to symmetry. We further analyze this effect in presence of
fabrication disorder and find that, even then, a pump suppression of close to
40 dB can be achieved in state-of-the art systems. Due to the small mode
volumes and high quality factors, our system is also expected to have a
generation efficiency much higher than in standard micro-ring systems. Those
two considerations make a strong case for the integration of our proposed
design in future on-chip quantum technologies.
