A team of Ohio University physicists thinks it has proposed a blueprint for building a better quantum dot. The researchers argue that defects formed during creation of the quantum dots operate as a barrier to scientific experimentation.

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.

The problem 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 temperature, creating an atomic coating. As more layers are added, the quantum dots bead up on the surface like droplets of water. 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.

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. “Now that they know the problem, they realize there are a few ways to avoid it,” Villas-Boas said.

The self-assembled type under study could be used in optical electronics and quantum computers. Other types, such as dots grown in a solution, might be used for solar energy applications. The study also will help the team better understand how to control the spin of electrons - a property that could be the underlying mechanism behind faster, more efficient future electronic devices.

Find out more at: http://nqpi.phy.ohiou.edu

A quantum dot (blue central bulge) being 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. (Photo: Jose Villas-Boas, Ohio University postdoctoral fellow)