By Roland Piquepaille
In this article, PhysicsWeb reports that researchers in the U.S. "have taken another important step towards making a quantum computer. [They] have created a logic gate using two electron-hole pairs -- also known as "excitons" -- in a quantum dot."Before going further, what is an exciton? Here is the definition from Wikipedia.
An exciton is a combination of an electron and a hole in a semiconductor or insulator in an excited state. The hole behaves as a positive charge, and the electron is attracted to it to form an "exotic atom" state akin to that of a hydrogen atom. The probability of the electron falling into the hole is limited by the difficulty of losing the excess energy, so that the exciton may have a relatively long life. The existence of these states may be inferred from the absorption of light associated with their excitation. Alternatively, an exciton may be thought of as an excited state of an atom or ion, the excitation wandering from one cell of the lattice to another.
So what exactly did these researchers?
[Duncan Steel of the University of Michigan] and co-workers grew a thin gallium arsenide layer 4.2 nm thick between two 25 nm aluminium gallium arsenide barriers to make a quantum dot. Electrons are trapped in the dot because the gallium arsenide layer has a smaller energy band-gap than the surrounding material. When excited by light, electrons from the valence band in the dot move to higher energy levels. The excited electron and the 'hole' it leaves behind combine to form an exciton. The system has four states: a ground state containing two unexcited electrons; two states containing one exciton; and a state containing two excitons.
For more information, visit this page about exciton transitions.
How this research will prove useful? Here are the conclusions of the author.
[The physicists] showed that the quantum-dot system behaves like a controlled-NOT gate in which the value of one qubit is reversed (the NOT operation) if - and only if - the value of the other qubit is 1.
Although it will not be possible to scale up the system, the group says that many of the ideas and techniques they have developed could be useful in other approaches to quantum computing based on the optical control of electron-spin qubits in quantum dots.
The research paper, "An All-Optical Quantum Gate in a Semiconductor Quantum Dot," by Xiaoqin Li, Yanwen Wu, Duncan Steel, D. Gammon, T. H. Stievater, D. S. Katzer, D. Park, C. Piermarocchi, and L. J. Sham, has been published by Science (Vol. 301, Pages 809-811).
Source: Belle Dumé, PhysicsWeb, August 8, 2003
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