Scientists find flaw in quantum dot construction

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Scientists find flaw in quantum dot construction
February 10, 2005

Nanoscientists dream of developing a quantum computer, a device the
size of a grain of sand that could be faster and more powerful than today's
PCs. They've identified tiny artificial atoms - called "quantum dots" - as
the most likely materials to build these machines, but have been puzzled by
the dots' unpredictable behavior at the nanoscale. Now a team of Ohio
University physicists thinks it's found the problem - and has proposed a
blueprint for building a better quantum dot. The researchers, who published
their findings in this week's issue of Physical Review Letters, 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. 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 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 new finding ultimately could lead to the creation of a better quantum
dot and can help scientists understand more about quantum states, Ulloa
added. "It's one more step towards the holy grail of finding a better
quantum bit, which hopefully will lead to a quantum computer," he said.

Nanoscientists are creating quantum dots in many different ways, Ulloa
noted, for use in various applications. 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 Ohio University 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, he added.
--
Ken

"Buddhism elucidates why we are sentient."