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Quantum Dots May Yield Quantum Changes in Computers

Like latter-day Thomas Edisons, researchers in the department of electrical engineering at the University of Nebraska–Lincoln have labored for years in search of just the right combination of elements.

And like Edison in his search to perfect the electric light bulb, if Supriyo Bandyopadhyay (ban-duh-pad-high) and his colleagues succeed, they will have a discovery that could revolutionize how the world works.

Bandyopadhyay’s Quantum Device Laboratory at NU is one of a handful of labs in the United States in the final round of two federal government funding competitions to develop quantum dot-based electronics. The Nebraska group has patented a technique that produces quantum dots, tiny structures that are 10,000 times smaller than the thickness of a human hair, but whose potential is staggering. In the next few decades, they could make binary computers obsolete and in a few years could make satellites safer from laser attacks.

“There are many applications for quantum dots,” Bandyopadhyay said. “The obvious application is that you can make very small structures and if you can store information in them, you can have very high information storage density. You can also use these structures to do very efficient, very high-speed computation. You can build quantum computers.”

“As an example of what quantum computers can do, let’s say that you wanted to build a computer that has two to the 1,000th power bits of data. You could never build a classical computer to do that because the number two to the 1,000th power is larger than the number of atoms in the known universe. With a quantum computer, all you would need is just 1,000 atoms to build a computer that powerful.”

In other words, a device far too small to be seen by the naked eye would vastly outstrip in power and speed any computer now in existence. It would also do it without generating heat and may be capable of storing data indefinitely. In contrast, conventional computers produce large amounts of heat and have to refresh their data several times a second to avoid losing it.

A quantum computer would be able to do all that because of how the laws of physics change at the atomic, or quantum, level.

“There are some fundamental properties of any quantum mechanical system, namely the ability of the quantum system to exist simultaneously in different states — the power to be in two different places at the same time, a phenomenon called ‘quantum parallelism,’” Bandyopadhyay explained. “You have to be able encode bits of information in the various states of the atoms in an appropriate way so that you can do quantum mechanical manipulations with them.”

Don’t expect to run down to the local Radio Shack to buy one anytime soon.

Bandyopadhyay and five fellow NU electrical engineers studying nanotechnology — Rod Dillon, Ned Ianno, Latika Menon, Paul Snyder and Frazer Williams — have been working on quantum computer research for about three years and have succeeded in demonstrating new types of computer memory. But Bandyopadhyay said his team is probably five years away from being able to demonstrate a small-scale quantum computer in the lab, while commercial versions probably won’t be available for 20 to 25 years.

Another breakthrough that Bandyopadhyay and colleagues are working on will probably be implemented much sooner.

Quantum dots can also be used to create high-speed, non-linear optical devices that would shield satellites from laser attack and improve the military’s abilities in electronic warfare, infrared imaging, night vision and surveillance.

“If you shine a light on an object, some of the light is reflected. Non-linear optics means that the fraction of the reflected light depends on the intensity of the incident light,” Bandyopadhyay said. “By changing the intensity of the incident light, you can change the fraction of the incident light that is reflected.

Supriyo Bandyopadhyay

Atomic Force micrograph of a porous alumite template self-assembles by anodizing an aluminum foil under the right electrochemical conditions. The pores are subsequently filled up with the material of choice by electrodeposition to create quantum dots. The template shown in this picture will create quantum dots of average diameter 50 nm. Forty billion such quantum dots will fit in an area the size of a postage stamp. Researchers in Electrical Engineering have made quantum dots as small as 3.5 nm in diameter. More than a trillion of such quantum dots have applications in high density computer memory, night vision, military intelligence gathering and quantum computing.

It's not all black and white

Optical micrograph of a Coulomb crystal produced in the Electrical Engineering department. The white particles have a diameter of less than 1 micrometer (100 times smaller than the thickness of a human hair). This technology produces nanostructures of material s that could not be handled by the electrochemical techniques because the latter is based on aqueous chemistry. Material that dissolve in water or otherwise reactwith water can be self-assembled into an ordered array of nanostructures using Coulomb crystallization which is a 'dry' technique.

These features can be produced on the surface of aluminium by electropolishing under different conditions. The structure on the top can be used to make quantum wires which have applications in optics, electronics and magnetics; the structure on the bottom can be used to make quantum dots. In 1997, the U.S. Army Research Office selected the discovery of the self-assembly process that produces these structuresas one of four most notable achievements in nanoscience. It was subsequently featured in the Army's "Nanoscience Poster".

“We set a world record in that by demonstrating the largest non-linear coefficients in some semiconductor quantum dots. We were able to do that by essentially making the structures very small and in a very well-ordered, regimented array. People get excited if they get a tiny percent increase and we got a 500 percent increase.”

At first glance, the NU team’s patented process for producing quantum dots is simplicity itself. Bandyopadhyay and his colleagues take a piece of aluminum, subject it to electro-chemical processes and the quantum dots spontaneously appear on the surface of the aluminum. But the devil is in the details. The chemical solution has to be just the right mixture, and the electrical current has to be at just the right power level and used for exactly the right length of time. Otherwise, the dots won’t arrange themselves in the perfectly ordered rows that Bandyopadhyay and his team need.

“There has been a whole lot of experimentation and it’s still going on,” Bandyopadhyay said. “We’ve been working on this for five or six years now and it’s far from perfect. Maybe in another five or six years we can perfect it.”

— Tom Simons