to optimal antibunching of the output signal. In particular, we have found that a close to ideal single-photon output can be obtained from asymmetric losses of the coupled modes, as it naturally occurs in photonic crystal molecules. The robustness of such a device is tested against unwanted loss channels, e.g. giving rise to pure dephasing of the normal modes.

[ Further analysis will be needed to engineer the output of the device as a useful single-photon wave packet for actual applications in integrated quantum circuits. ]{} However, the results presented here provide a useful starting ground to design highly innovative quantum photonic devices based on integrated sources of pure single-photon states. The main innovations come from the possibility to employ commonly nanostructured materials from optoelectronic and CMOS-compatible industry, working at room temperature, and in the telecom band for long distance low-loss data transmission. We believe this work will further stimulate research in this direction, also in view of recent experimental demonstrations of enhanced native optical nonlinearities in semiconductor-based photonic crystal platforms [@derossi2008; @galli2010; @graphene2012].

Acknowledgements {#acknowledgements .unnumbered}
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We acknowledge Timothy C. H. Liew for contributing to the early stages of these calculations, and for useful comments and suggestions, and 