Quantum dots

The ultimate ‘zero-dimensional’ magnetic nanostructure is a dot so small that it contains few electrons, or maybe even a single one. The capacitance of a sphere of radius r is C = 4π∈₀r. The potential of a single electron on the sphere is V = e/C; for example, if r = 14.4 nm, V = 100 mV. This Coulomb barrier to adding charge to the nanodot capacitor is known as Coulomb blockade. The electron content of the quantum box can be controlled by tunelling electrons, one at a time, by adjusting the bias of the adjacent electrodes, which may be ferromagnetic.
The quantum dot is really an artificial atom, with a square-well potential rather than a Coulomb potential. At low temperature the unpaired spin moment of the dot can form a Kondo singlet state with electrons in nonmagnetic electrodes. Spin-polarized electron flow across it can be regulated by adjusting the potential of a gate. This is a magnetic single-electron transistor. Pairs of these magnetic quantum dots are candidate q-bits for a quantum computer.