Duke, Purdue Scientists Control Two 'Quantum Dots'

Scientists from Duke and Purdue universities have fabricated a complex transistor that can control two minuscule dots of electrons so tightly that individual electrons from each dot can interact through the exotic rules of quantum mechanics.

This achievement represents an early step in efforts to create quantum mechanical computers, said one of the scientists, Duke physics professor Albert Chang.

"For the market, quantum computers mean better encryption methods and heightened data security," Chang said in an interview. "For science, our research may help address the longstanding mystery of the relationship between the classical physics of the world we see every day and the peculiar world of quantum physics that governs the tiny particles inside atoms."

 

In a paper published in the April 30, 2004, edition of the journal Physical Review Letters, Chang and two other scientists reported they had "tuned" single electrons confined in each of two microscopic "quantum dots" under the transistor's surface to act like the electrons were coupled together even though they were actually separated by a very small space.

 

In principle, these electrons would have continued to act in synchrony even if one of them were transported a continent's distance apart, said Chang. Since in essence they could "communicate" at both large and small distance those electrons were thus considered to be "entangled," a quantum mechanical property.

 

The lead author of the Physical Review Letters paper was Jeng-Chung Chen, who received his doctorate at Purdue under Chang and is now at the University of Tokyo. Co-authors are Chang, who in 2003 relocated from Purdue to Duke, and Michael Melloch, a professor at Purdue's School of Electrical and Computer Engineering.

 

The research was supported by the National Science Foundation.

 

Entangled electrons could serve as quantum bits, or qubits, in a futuristic computer employing quantum rules that can work exceptionally fast for some kinds of calculations, Chang said. Entanglement could also ensure that the secret code to unraveling encrypted messages remains secret.

 

A quantum dot, sometimes called an artificial molecule or atom, is a grouping of several million atoms whose electronic properties allow for several dozen individual electrons to be corralled within a "puddle," Chang said.

 

The Purdue-Duke team was able to create two quantum dots in close proximity by crafting an unusually complex field effect transistor fabricated out of the semiconductors gallium arsenide and aluminum gallium arsenide. Each dot had a width of only about 200 nanometers, one nanometer measuring one billionths of a meter. It would take about 5,000 dots placed end-to-end to stretch across a sand grain, he estimated.

 

Eight separate "gates," features that control the flow of current in regular field effect transistors, were configured on the device's surface so that they converged on both dots. This arrangement allowed the researchers to control the electronic properties of each dot so tightly that electrons could be admitted or expelled one by one.

 

Electrons come in two different spin states, either "up" or "down." And a single electron in one spin state naturally pairs with another of the opposite spin.

 

After corralling 40 to 60 of such paired electrons in each dot, the researchers added another single unpaired electron to each set. Adding an extra electron imparts a net spin of up or down to the entire quantum dot. And the researchers were able to arrange the electron grouping so that both dots had the same net spin.

 

"When isolated from one another, the two net spins would not seek to pair with each other," Chang said. "But we have a special method of 'tuning' the two-dot system so that, despite the similar spins, the two unpaired electrons became entangled - they began to interact with one another.

 

"Entanglement is a key property that would help give a quantum computer its power," he said. "Because each system exists in this mixed, down-up configuration, it may allow us to create switches that are both on and off at the same time. That's something current computer can't do."

 

Using the language of computer scientists, the up spin would represent a 1 and a down spin a 0 for each individual qubit, for possible use in memory chips. Large groups of qubits could be used to solve problems that have myriad potential solutions that must be winnowed down quickly, such as factoring the very large numbers used in data encryption, he said.

 

"A desktop computer performs single operations one after another in series. It's fast, but if you could do all these operations together, in parallel rather than in series, it can be exponentially faster.

 

"In the encryption world, solving some problems could take centuries with a conventional computer. But for a quantum computer, whose bits can be in two quantum states at once -- both on and off at the same time - many solutions could, in theory, be explored simultaneously, allowing for a solution in hours rather than lifetimes."

 

Chang cautioned that his group's current achievement falls far short of a quantum computer. "Essentially, what we've done is just a physics experiment, no more," he said. "In the future, we'll need to manipulate the spin at very fast rates. But for the moment, we have for the first time demonstrated the entanglement of two quantum dots and shown that we can control its properties with great precision.

 

After relocating his laboratory to Duke, Chang is now continuing his experiments, which are done at extremely frigid temperatures below those at which helium liquefies.

 

The low temperatures slow down the movements of atoms and, to a lesser degree, electrons. Such lethargy is necessary to observe properties that follow the rules of quantum mechanics, he explained.