Quantum computing breakthrough arises from unknown molecule

June 26, 2008

WEST LAFAYETTE, Ind. - The odd behavior of a molecule in an experimental
silicon computer chip has led to a discovery that opens the door to
quantum computing in semiconductors.

In a Nature Physics journal paper currently online, the researchers
describe how they have created a new, hybrid molecule in which its
quantum state can be intentionally manipulated - a required step in the
building of quantum computers.

"Up to now large-scale quantum computing has been a dream," says Gerhard
Klimeck, professor of electrical and computer engineering at Purdue
University and associate director for technology for the national
Network for Computational Nanotechnology.

"This development may not bring us a quantum computer 10 years faster,
but our dreams about these machines are now more realistic."

The workings of traditional computers haven't changed since they were
room-sized behemoths 50 years ago; they still use bits of information,
1s and 0s, to store and process information. Quantum computers would
harness the strange behaviors found in quantum physics to create
computers that would carry information using quantum bits, or qubits.
Computers would be able to process exponentially more information.

If a traditional computer were given the task of looking up a person's
phone number in a telephone book, it would look at each name in order
until it found the right number. Computers can do this much faster than
people, but it is still a sequential task. A quantum computer, however,
could look at all of the names in the telephone book simultaneously.

Quantum computers also could take advantage of the bizarre behaviors of
quantum mechanics - some of which are counterintuitive even to
physicists - in ways that are hard to fathom. For example, two quantum
computers could, in concept, communicate instantaneously across any
distance imaginable, even across solar systems.

Albert Einstein, in a letter to Erwin Schrödinger in the 1930s, wrote
that in a quantum state a keg of gunpowder would have both exploded and
unexploded molecules within it (a notion that led Schrödinger to
create his famous cat-in-a-box thought experiment).

This "neither here nor there" quantum state is what can be controlled in
this new molecule simply by altering the voltage of the transistor.

Until now, the challenge had been to create a computer semiconductor in
which the quantum state could be controlled, creating a qubit.

"If you want to build a quantum computer you have to be able to control
the occupancy of the quantum states," Klimeck says. "We can control the
location of the electron in this artificial atom and, therefore, control
the quantum state with an externally applied electrical field."

The discovery began when Sven Rogge and his colleagues at Delft
University of Technology in the Netherlands were experimenting with
nano-scale transistors that show the effects of unintentional
impurities, or dopants. The researchers found properties in the
current-voltage characteristics of the transistor that indicated
electrons were being transported by a single atom, but it was unclear
what impurity was causing this effect.

Physicist Lloyd Hollenberg and colleagues at the University of Melbourne
in Australia were able to construct a theoretical silicon-based quantum
computer chip based on the concept of using an individual impurity.

"The team found that the measurements only made sense if the molecule
was considered to be made of two parts," Hollenberg says. "One end
comprised the arsenic atom embedded in the silicon, while the
'artificial' end of the molecule forms near the silicon surface of the
transistor. A single electron was spread across both ends.

"What is strange about the 'surface' end of the molecule is that it
occurs as an artifact when we apply electrical current across the
transistor and hence can be considered 'manmade.' We have no equivalent
form existing naturally in the world around us."

Klimeck, along with graduate student Rajib Rahman, developed an updated
version of the nano-electronics modeling program NEMO 3-D to simulate
the material at the size of 3 million atoms.

"We needed to model such a large number of atoms to see the new,
extended quantum characteristics," Klimeck says.

The simulation showed that the new molecule is a hybrid, with the
naturally occurring arsenic at one end in a normal spherical shape and a
new, artificial atom at the other end in a flattened, 2-D shape. By
controlling the voltage, the researchers found that they could make an
electron go to either end of the molecule or exist in an intermediate,
quantum, state.

This model was then made into an image by David Ebert, a professor of
electrical and computer engineering at Purdue, and graduate student
Insoo Woo.

Delft's Rogge says the discovery also highlights the current
capabilities of designing electronic machines.

"Our experiment made us realize that industrial electronic devices have
now reached the level where we can study and manipulate the state of a
single atom," Rogge says. "This is the ultimate limit, you can not get
smaller than that."

http://news.uns.purdue.edu/x/2008a/080626KlimeckArsenic.html
<http://news.uns.purdue.edu/x/2008a/080626KlimeckArsenic.html>


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