Artificial atom

From ArticleWorld

Artificial atom is the term used to denote objects that possess discrete electronic states, on the same lines as naturally occurring atoms.

Artificial atoms – The underlying concept

The most common example of artificial atoms is seen in quantum dots. A quantum dot is essentially a semiconductor nanostructure that places confinements on the motion of conduction band electrons, valence band holes and electron-hole pairs (also termed excitons). This confinement can be made possible due to electrostatic potentials that are ensured by the external electrodes present in the system or by the process of doping the semiconductor material with impurities. The presence of a semiconductor surface can also ensure this confinement.

Thanks to modern techniques of lithography, it has become easy to confine electrons to such small dimensions that the quantization of both their charge and energy are easily observable. Such confined electrons are allowed to tunnel their way to electrodes, a phenomenon which explains the working of a single electron transistor (SET). SETs, being closely analogous to atoms in their internal structure, are easy to study and implement.

The single electron transistor turns on and off every time an electron is added to it. Such a device is completely quantum mechanical. Artificial atoms such as these are observed by taking measurements of voltage and current that results because of tunneling between them and electrodes in their proximity.

A typical artificial atom – The SET

An SET typically consists of a semiconductor like gallium arsenide which is separated from metal electrodes by an insulator like aluminum gallium arsenide. This insulator, which is doped with silicon, is capable of donating electrons. The structure is grown with molecular beam epitaxy and electrodes are fabricated with electron beam lithography. A two-dimensional electron gas is created at the interface of the semiconductor and the insulating layer, as a result of the positive charge on the silicon atoms.

Just as a field effect transistor (FET) can act as a switch and turn on and off, an artificial atom uses the electron spin phenomenon to act as a switch.

Research on artificial atoms

There has been considerable research being carried out in this domain over the past few decades. Studies in major research centers mainly focus on developing ways to improve practicality of the concept of artificial atoms. The use of the phenomenon of electron spin in atoms may soon make quantum memories in computers a reality, drastically reducing sizes.