Universal self-organization and self-ordering effects at surfaces of semiconductors lead to the formation of coherent zero-dimensional clusters, quantum dots (QDs). The electronic and optical properties of QDs, being smaller than the de-Broglie-wavelength in all three directions of space are close to those of atoms in a dielectric cage than of solids. Their delta-function-like energy eigenstates are only twofold (spin) degenerate. All few particle excitonic states are strongly Coulomb correlated due to the strong carrier localisation. Their energies depend on shape and size of the dots, such that positive, zero or negative biexciton binding energies and finestructure splitting (caused by exchange interaction) appear.

Consequently, single QDs present the most practical possible basis of emitters of single polarized photons (Q-bit emitters) on demand or entangled photons via the biexciton-exciton cascade for future quantum cryptography, repeaters and communication systems. Embedding them in electrically pumped resonant cavity structures, they can emit single photons at rates beyond 1 Gbit/s. Using GaN-based structures room temperature operation is possible.

Multiple QD layers, as active materials for nano-optoelectronic devices like edge and surface emitting lasers, or semiconductor optical amplifiers, are extremely promising. Their properties, in particular their energy efficiency, are outperforming those of photonic devices based on higher dimensional systems.

Semiconductor nanotechnologies transform presently to enabling technologies for new economies. The commercialization of nano-devices and systems has started. High bit rate and secure quantum cryptographic system, nano-flash memories, ultra-high speed nano-photonic devices for metropolitan area networks, the 400 Gbit/s Ethernet,… present some of the first fields of applications of nano-devices.