Self-assembled quantum dots in a nanowire system for quantum photonics. 
Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics.  Whereas the top-down fabrication of such structures remains a technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy.  However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult.  We present a versatile quantum-dot-in-nanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires.  The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre.  Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core.  Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers.  By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells.