By Roland Piquepaille
Many teams of nanoscientists around the world want to be the first one to build quantum computers. To achieve this goal, they're using artificial atoms -- also known as 'quantum dots.' But even if they're able to use them, not a single team has been able to consistently control their quantum mechanical states -- or their properties -- at the nanoscale. Now, a team from Ohio University claims it found a flaw in quantum dot construction and proposes a solution. And guess what? As it happens often in research, this new finding is based on a very simple fact: an interference between two physical phenomena. Read more...[For example,] experimental scientists in Germany had blasted the quantum dots with light to create the quantum mechanical state needed to run a quantum computer. But they couldn't consistently control that state, explained Sergio Ulloa, an Ohio University professor of physics and astronomy. Jose Villas-Boas, a postdoctoral fellow at Ohio University, Ulloa and Associate Professor Alexander Govorov developed theoretical models to learn what went wrong.
The problem, they argued, happens during the creation of the type of quantum dots under study. Using a molecular beam epitaxy chamber, scientists spray paint a surface with atoms under high temperatures, creating an atomic coating. As more layers are added, the quantum dots bead up on the surface like droplets of water, Ulloa said.
But a fine residue left behind on the surface that Ulloa calls the "wetting layer" can cause problems during experiments. When experimental scientists blasted the quantum dots with a beam of light in previous studies, the wetting layer caused interference, instead of allowing the light to enter the dot and trigger the quantum state, he explained.
The illustration above "shows a quantum dot (blue central bulge) bombarded from the top with laser light. The laser produces excitations (called excitons) inside the dot, and the electric fields generated by the top and bottom gold contacts pull the electrons (yellow) and holes (red) away. Other electrons/holes are undesirably produced instead on the wetting layer, causing interference. The semiconductor compounds used in these experiments are Gallium Arsenide (GaAs) and Indium Gallium Arsenide (InGaAs)." (Credit for image and legend: Jose Villas-Boas) Here is a link to a larger version.
The study suggests that scientists could tweak the process by re-focusing the beam of light or changing the duration of the light pulses to negate the effects of the wetting layer, Villas-Boas said. One experimental physicist already has used the theoretical finding to successfully manipulate a quantum dot in the lab, he added. "Now that they know the problem, they realize there are a few ways to avoid it," Villas-Boas said.
The research work from these scientists at the Ohio University's Nanoscale & Quantum Phenomena Institute has been published in a recent issue of Physical Review Letters on February 8, 2005 under the name "Decoherence of Rabi Oscillations in a Single Quantum Dot." Here is a link to the abstract.
We develop a realistic model of Rabi oscillations in a quantum-dot photodiode. Based in a multiexciton density matrix formulation we show that for short pulses the two-level model fails and higher levels should be taken into account. This affects some of the experimental conclusions, such as the inferred efficiency of the state rotation (population inversion) and the deduced value of the dipole interaction. We also show that the damping observed cannot be explained using constant rates with fixed pulse duration. We demonstrate that the damping observed is in fact induced by an off-resonant excitation to or from the continuum of wetting layer states. Our model describes the nonlinear decoherence behavior observed in recent experiments.
And if you want to know more, but don't want to buy the article, here is a link to the full article (PDF format, 5 pages, 221 KB), thanks to the invaluable arXiv.org website.
Finally, just in case you wouldn't know anything about Rabi oscillations, please read Isidor Isaac Rabi's biography. He won the Nobel Prize for Physics in 1944.
Sources: Andrea Gibson, Ohio University, via EurekAlert!, February 10, 2005; and various websites
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