Quantum Dots as Building Blocks for High-Security Computers

Michael A. Greenwood

The allure of a quantum computer is powerful. The device, theoretically, could process certain types of information at such an accelerated pace that the classical computers now in use might seem in comparison as antiquated as the manual typewriter.

Researchers seeking to build such an advanced system believe that quantum dots — epitaxially grown semiconductor nanocrystals about 10 nm in diameter — have the optical properties needed to power the next generation of supercomputers.

However, controlling the electron spin of a quantum dot is a key step in powering an optically driven quantum computer. The spin of the electron would form the computing system’s qubits (as opposed to bits, the foundation of current computing systems) and is believed to have the long quantum coherence time necessary to perform quantum manipulations.

Dual lasers

Investigators from the University of Michigan in Ann Arbor, from the University of California, San Diego, and from the US Naval Research Laboratory in Washington recently used dual lasers to control and change the spin of an electron in a single quantum dot.

Lead researcher Duncan G. Steel of the University of Michigan said that this type of control has never been achieved with a semiconductor system. The researchers used indium-arsenide self-assembled quantum dots embedded in a Schottky diode structure that allowed the quantum dots to be biased to have a net charge of exactly one electron. They mixed the fields from two lasers, a diode laser from Sacher Lasertechnik and a Ti:sapphire laser from Coherent Inc. of Santa Clara, Calif., onto the quantum dot and measured the response. Indium-arsenide quantum dots were selected because of their high optical and electronic quality.

This figure shows a typical spectrum of emission energy (of the photon in photoluminescence) as a function of applied bias voltage in a coupled quantum dot system of the type being engineered for a scalable quantum logic device. The distinctive feature seen here is the electronic states of two separate quantum dots and the junctions they form. These junctions could be coupled to form entanglements, the foundation of a quantum computing system. Courtesy of Dan Gammon, US Naval Research Laboratory, Washington.

The experiments showed that the state of a qubit could be changed with only 10^–18 J of energy at a rate in excess of 1.4 GHz, Steel said. Based on the electronic structure of the quantum dot, the researchers believe that the upper limit is probably closer to 100 GHz.

Anticipated applications for such a device include national security. It is expected that a quantum computer, a fraction the size of a classical computer, would be able to construct, as well as to decipher, complex encryption codes within a few seconds, light-years ahead of what computers can do now. The technology also could be readily transferred to everyday security applications such as credit card protection.

Steel said that one intriguing aspect of the experiments is that the high-performance lasers used in the laboratory were not even necessary. Lasers used in ordinary telecommunications technology could be used to drive a quantum dot.

The next step will be to take two quantum dots and simultaneously control the spin of one electron in each. In this experiment, investigators controlled the state of only one quantum dot. If each spin can be independently controlled, a swap gate could be created to produce quantum entanglements. Researchers believe that the entangled states will allow the enhanced performance of a quantum computer.

In the meantime, engineering quantum dots that can be coupled into arrays of two or more remains a challenge before even a basic quantum computing device can be created. Steel estimated that it could take another decade before simple demonstrations of mathematical/physical interest could be performed.

Science, Aug. 17, 2007, pp. 929-932.