  Non-classical light sources offer a myriad of possibilities in fundamental
science and applications including quantum cryptography and quantum
lithography. Single photons can encode quantum information and multi-qubit
gates in silica waveguide circuits have been used to demonstrate linear optical
quantum computing. Scale-up requires miniaturisation of the waveguide circuit
and multiple photon sources. Silicon photonics, driven by the incentive of
optical interconnects, is a highly promising platform for the passive
components, but integrated light sources are limited by silicon's indirect
band-gap. III-V semiconductor quantum-dots, on the other hand, are proven
quantum emitters. Here we demonstrate single-photon emission from quantum-dots
coupled to photonic crystal nanocavities fabricated from III-V material grown
directly on silicon substrates. The high quality of the III-V material and
photonic structures is emphasized by observation of the strong-coupling regime.
This work opens-up the advantages of silicon photonics to the integration and
scale-up of solid-state quantum optical systems.
