The term "Quantum Dot" is applied to a broad range of man-made, solid-state structures, but they are all characterised by the existence of localised discrete quantum states, as opposed to the continuum of states that typically occur in macroscopic systems (i.e. objects consisting of many atoms). For this reason, they are often called artificial atoms, and their properties may be engineered to suit different applications.
Quantum dots may be classified in a number of different ways, one of which is the degree of freedom chosen to represent the qubit: we can have spin qubits, where the electron spin-1/2 represents the qubit, or charge qubits, where the charge (orbital) degree of freedom serves the purpose.
Another way of classifying quantum dots is according to their method of construction, e.g. self-assembled, implanted or electrostatically defined. In the following we will briefly describe these three methods.
Self assembled quantum dots are formed in heterogeneous systems, when one kind of semiconductor material is deposited by chemical vapour deposition onto the surface of another semiconductor with different lattice spacing, e.g. Indium Gallium Arsenide deposited onto Indium Gallium Phosphide. The difference in the lattice spacing means that it is energetically favourable for the secondary crystal to form localised structures, rather than a flat layer. This is closely analogous to the way mercury droplets do not disperse when placed on a surface.
Implanted quantum dots are formed when a few (perhaps just one) dopant atoms are deliberately placed in an otherwise perfect crystal lattice. For a shallow donor, the outer electron from the donor is weakly bound to the donor atom, and so looks rather like a hydrogen atom, but in a crystal medium, rather than in a vacuum. A deep donor has a strongly bound electron. Acceptor dopant atoms can also be considered. A commonly studied system is isotpically pure Si-28 doped with a P-31 atom, which forms the basis of the Kane proposal for solid-state spin-based quantum computing.
Electrostatically defined dots are constructed by first making a two-dimensional gas of electrons (which can be formed at the interface between two different crystalline compounds such as AlGaAs/GaAs), and then depleting selected regions using surface electrodes to repel electrons from the region under the electrodes. Gaps in the electrode structure form localised potential wells into each of which a single (or a few) electrons can be trapped. Other surface electrodes can manipulate the potential wells controlling their individual energies and the tunneling rates between them.
All of these structures have discrete energy levels, which are characterised by their typical energy separations: self-assembled dots are typically optically active structures, whose energy spacings are of the order of 1 eV. Other kinds of dots have energies of a few μeV to a few meV, so must be operated at very low temperatures.