quantum computation is now practical

[Kragen Sitaker]

>From http://www.eet.com/at/news/OEG20020806S0030.  The highlight is
this paragraph from the end of the article:

    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.


Quantum computer called possible with today's tech
--------------------------------------------------

By R. Colin Johnson
EE Times
August 7, 2002 (6:36 a.m. EST)

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.


-- 
<kragen@pobox.com>       Kragen Sitaker     <http://www.pobox.com/~kragen/>
Edsger Wybe Dijkstra died in August of 2002.  This is a terrible loss after 
which the world will never be the same.
http://www.xent.com/pipermail/fork/2002-August/013974.html