design aims for quantum leap
Technology Research News
The first step toward making phenomenally
powerful quantum computers is capturing and manipulating individual subatomic
particles, which is a bit like getting a fly to venture onto your desk,
then perform tricks like "sit up" and "roll over" on command.
The second step is harnessing, controlling and coordinating thousands
or millions of particles at once. Making a practical quantum computer
also means doing this using ordinary electronics rather than exotic laboratory
University of Wisconsin researchers are tackling these issues with a quantum
computer design that would incorporate thousands of individually-controlled
electrons into a silicon chip that could be made much the same way as
today's computer chips.
Practical quantum computers would be many orders of magnitude faster than
today's computers for problems that involve massive amounts of data, like
cracking secret codes and searching large databases.
The researchers' idea is to "trap single electrons in tiny silicon sandwiches
about a millionth of an inch across," said Robert Joynt, a physics professor
at the University of Wisconsin at Madison. The silicon sandwiches are
quantum dots, microscopic specks of semiconductor material that can hold
one or a few electrons.
These dots can represent bits of information because an electron acts
like a tiny spinning top, and depending on which way it is spinning it
can represent a 1 or a 0. Conventional computers use the presence or absence
of electric current running through transistors to indicate the 1s and
0s of digital information.
Proposals for making quantum computers out of quantum dots have been around
for several years. The Wisconsin researchers' design plots out some of
the difficult details -- it allows individual electrons to be loaded into
the quantum dots and allows interactions between electrons held in neighboring
dots is to be closely controlled.
Each dot would consist of a bottom layer of silicon germanium that has
been chemically altered to allow electrons to flow more easily. This layer
would serve as a reservoir of electrons.
The middle layers would consist of an extremely thin layer of silicon
sandwiched between layers of unaltered silicon germanium. The silicon
layer would hold the individual electron used by the quantum computer,
and the silicon germanium layers would act as barriers to keep additional
electrons out. The researchers could coax individual electrons to tunnel
through the barriers to the silicon layer by changing the electrical current
running through the chip.
Metal electrodes that move the electrons laterally would form the chip's
top layer. The electrodes would be used to bring pairs of electrons in
adjacent dots together to perform the basic logic operations of computing.
The quantum interactions of a pair of electrons can be represented mathematically,
and that math can be used to generate the binary logic that is the foundation
of computing. This allows logic operations like adding binary numbers
to be carried out by controlling the electron interactions.
"The biggest hurdle is fabrication," said Joynt. "This needs to be done
with exquisite control of the quality of the material and to very high
measurement specs," he said.
Because the quantum dots are made from layers of metal and semiconductors,
like computer chips, the researchers' proposed device could be built using
standard chipmaking processes, according to Joynt. "The dots are only
slightly smaller than the features on commercial chips, which have millions
of transistors," he said.
Unless the optical lithography used in the commercial chip industry improves,
however, this minor decrease in size means that the researchers will have
to use electron beam lithography, said Joynt. "This is slower and more
expensive, but perhaps not prohibitively so," he said.
Today's optical lithography uses ultraviolet light with wavelengths ranging
from 200 to 300 nanometers and can etch features as small as 130 nanometers.
Electron beams can be focused with magnetic fields to around 10 nanometers
and so can etch much smaller features.
The potential benefits of a practical quantum computer are enormous.
"Quantum computing is massively parallel," said Joynt. This means that
many inputs can be processed at the same time, which makes for a computer
that can solve problems that would take a regular computer "essentially
forever" to work out, he said.
When an electron is isolated from its environment it is in the weird quantum
state of superposition, meaning it is spinning in both directions at once.
An electron in superposition can represent a mix of 1 and 0, and a string
of electrons in superposition can represent every combination of 1s and
0s at the same time.
The power of a quantum computer comes from the ability to check every
possible combination of numbers at once to find the answer to a problem
that can have more possibilities than there are atoms in the universe.
Ordinary computers have to check each possible answer one at a time.
Researchers have already come up with software that would allow quantum
computers to crack secret codes and search massive databases.
The Wisconsin work is a good effort that adds "many realistic details"
to quantum dot research, said IBM Research physicist David DiVincenzo.
DiVincenzo and Daniel Loss, a physics professor at University of Basel
in Switzerland, developed an earlier quantum dot quantum computer proposal.
"I am very encouraged generally by the efforts of the University of Wisconsin
group," said DiVincenzo. "They have started a big, integrated effort involving
both theory and experiment," he said.
Practical quantum computers are likely to take 25 years to develop, said
Joynt. "And I'm an optimist," he said. "We are working on fabricating
a prototype, step by step," he added. "The next step is to make sure that
our [silicon layer] is properly trapping the electrons."
Joynt's research colleagues were Mark Friesen, Paul Rugheimer, Donald
Savage, Max Lagally, Daniel van der Weide and Mark Eriksson. They published
the research in the July 15, 2002 issue of the journal Physical Review
B. The research was funded by the U.S. Army Research Office (ARO) and
the National Science Foundation (NSF).
Timeline: 25 years
TRN Categories: Quantum Computing and Communications
Story Type: News
Related Elements: Technical paper, "Design and proof of
concept for silicon-based quantum dot quantum bits," posted on the arXiv
physics preprint archive at arXiv.org/abs/cond-mat/0204035; Technical
paper, "Decoherence of electron spin qubits in Si-based quantum computers,"
Physical Review B, July 15, 2002
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