ATHENS,
Ohio – 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.
The research was supported by grants from the Department
of Energy, the Indiana 21st Century Fund, the Ohio
University Postdoctoral Fellow Program, and the FAPESP
fellowship. The researchers are members of Ohio University's
Nanoscale and Quantum Phenomena Institute, http://nqpi.phy.ohiou.edu/.
Attention
For a copy of the paper on which this story is based,
visit the Physical Review Letters Web site at http://prl.aps.org/
or contact either the authors below or Andrea Gibson,
(740) 597-2166, gibsona@ohio.edu.
Contacts: Jose Villas-Boas,
(740) 593-9611, villasb@phy.ohiou.edu; Sergio Ulloa,
(740) 593-1729, ulloa@ohio.edu
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