Quantum computers inch closer?

[Perry E. Metzger]

[I don't know what to make of this story. Anyone have information? --Perry]

Quantum computer called possible with today's tech
http://www.eet.com/story/OEG20020806S0030

MADISON, Wis.   Researchers at the University of Wisconsin in
Madison claim to have created the world's first successful simulation
of a quantum-computer architecture that uses existing silicon
fabrication techniques. By harnessing both vertical and horizontal
tunneling through dual top and bottom gates, the architecture lays
out interacting, 50-nanometer-square, single-electron quantum dots
across a chip.

"Our precise modeling elucidates the specific requirements for
scalable quantum computing; for the first time we have
translated the requirements for fault-tolerant quantum computing into
the specific requirements for gate voltage control electronics in
quantum dots," said professor Mark Eriksson of the university's
Department of Physics. 

The group of researchers has concluded that existing silicon
fabrication equipment can be used to create quantum computers, albeit
at only megahertz speeds today due to the stringent requirements of
its pulse generators. To achieve gigahertz operation, the group has
pinpointed the device features that need to be enhanced to prevent
leakage errors, and has already begun work on fabricating a
prototype. 

"We believe that quantum computers are possible today with the
component technologies we already have in place for silicon,"
Eriksson said. The team composed their quantum "bits" out of electron
spin: up for "1," down for "0." Encoding bits in spins allows a
single electron to represent either binary value, and because of the
indeterminacy of quantum spins, they can represent both values during
calculations to effectively create a parallel process.

"Our technique may enable quantum computers to actually begin
performing calculations that can't be performed any other way,"
Eriksson said. Others have demonstrated a few quantum dots
interacting to perform calculations but Eriksson estimates that a
million quantum bits (qubits) will be needed to create quantum
computers that perform useful real-world applications. For that,
silicon fabrication equipment offers the best solution, according to
Eriksson. 



Eriksson's team matched silicon germanium fabrication capabilities to
quantum-dot requirements. The result is an array of quantum dots,
each of which houses a single electron, with electrostatic gates
controlling qubit interactions. The team then optimized and
exhaustively simulated the model, which it declared to be a
successful design.

The design constraints included reducing the population of electrons
in quantum dots to one, while permitting tunable coupling between
neighboring dots. The team met those conditions by employing both
vertical and horizontal tunneling to first confine and then slightly
alter the location of individual electrons.

A back gate serving as the chip substrate acts as an electron
reservoir from which quantum dots can draw their single electrons
using vertical tunneling into the quantum-well layer. That layer acts
as the vertical confinement barrier, with an insulator above and
below it, enabling the vertical size of the quantum dots to be just
big enough for one. A grid of top gates then provides the horizontal
separation between dots by supplying electrostatic repulsion from
above.

The semiconductor layers were formed from strain-relaxed SiGe, except
for the quantum-well layer, which was pure, strained silicon. The
bottom gate was formed from a thick n-doped layer with a 10-nm,
undoped tunneling barrier separating it from the 6-nm-thick
quantum-well layer. Another 20-nm-thick tunnel barrier above the
quantum-well layer separated it from the metallic top gates, the team
reported.

Researchers load the electrons into the quantum dots from below by
adjusting the potentials on the top gates to induce an electron from
the bottom gate to tunnel vertically up into the quantum-well layer.
Once loaded, the electron stays in place because of the electrostatic
force from the top gates. When the team weakens the force between
selected quantum dots by adjusting the top gates between them, the
adjacent dots are permitted to interact, thus enabling calculations
to be made.

The normal errors encountered during quantum calculations could
mostly be corrected, according to Eriksson's simulations. Careful
consideration of the simulations led the researchers to predict that
leakage could be tuned out sufficiently by low temperatures combined
with a modified heterostructure that allowed larger electrical
fields. 

With existing fabrication techniques, the team estimates that a
million-quantum-dot computer (1,024 x 1,024 array) could be built
today and operated in the megahertz range. 


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