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Quantum computer keeps it simple

Quantum computer keeps it simple - Physics Forum

Quantum computer keeps it simple - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.

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Old 08-14-2003, 01:19 PM
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Default Quantum computer keeps it simple

Quantum computer keeps it simple
August 13/20, 2003
By Eric Smalley, Technology Research News

Quantum computers promise to be fantastically fast at solving certain
problems like cracking codes and searching large databases, which provides
plenty of incentive for overcoming the tremendous obstacles involved in
building them.

The basic component of quantum computers, the qubit, is made from an
atom or subatomic particle, and quantum computers require that qubits
exchange information, which means the interactions between these absurdly
tiny objects must be precisely controlled.

Researchers from the University of Oxford and University College
London in England have proposed a type of quantum computer that could
greatly simplify the way qubits interact.

The scheme allows qubits to be constantly connected to each other
instead of repeatedly connected and disconnected, and it allows a computer's
qubits to be controlled all at once, said Simon Benjamin, a senior research
fellow at the University of Oxford in England. Global control is a fairly
unconventional idea that "allows you to send control signals to all the
elements of the device at once instead of having to separately wire up each
element," he said.

The scheme can be implemented with different types of qubits. A common
type uses the spin of an electron. Electrons can be oriented in one of two
directions, spin up and spin down. These are analogous to the poles of a
kitchen magnet and can represent the 1s and 0s of computer information.

Key to the potential power of quantum computers is a weird trait of
quantum particles like electrons. When an electron is isolated from its
environment, it enters into superposition, meaning it is in some mix of both
spin up and spin down.

Linking two qubits that are in superposition makes it possible for a
quantum computer to examine all of the possible solutions to a problem at
once. But controlling how two qubits interact is extremely challenging, said
Benjamin. Qubits "must be made to talk to each other, and when the operation
is over they must be made to stop talking," he said.

In traditional quantum computing schemes that use electron spins,
pairs of qubits have a metal electrode between them. When the electrode is
negatively charged, it repels the negatively charged electrons that make up
the qubits, keeping them separated. But giving the electrode a positive
charge draws the electrons toward each other, allowing them to interact by
exchanging energy. Allowing the qubits to interact for half the time it
takes to completely swap energy is the basis of two-qubit logic gates.

The energy of the two qubits has to be resonant or errors can arise,
but off-resonant energy can also be harnessed, said Benjamin. Particles
resonate at specific energies in the same way that larger objects vibrate
more readily at certain frequencies. Different energies can be more or less
resonant with each other much like certain musical notes sounding better
together than others. "Something that we were used to thinking of as a
source of error could in fact be a means of controlling the computer," he

The researchers' proposal replaces the electrode with a third
electron. These three electrons are constantly interacting, but they don't
always exchange energy. When the middle electron is off resonant, the qubits
are blocked from exchanging energy. This way, the interaction "is always on,
but we can effectively negate it by ensuring that the energies of
neighboring spins are completely incompatible," said Benjamin.

Avoiding electrodes is useful for several reasons. Fabricating qubits
with electrodes between them "will require a fantastic degree of control,"
said Benjamin. "If a particular pair of electrons are too close, then the
interaction will be jammed on, and if they are too far away then the
interaction will be jammed off," he said.

Electrodes can also knock qubits out of superposition. "Each electrode
can act as an [antenna], channeling electromagnetic noise from the
room-temperature world right down to the qubits," said Benjamin.

The researchers took their proposal a step further by removing the
need to control electrons individually. Every change to the energy of the
electrons is applied to the whole device. The researchers divide a string of
qubits into two groups, odd and even, with every other qubit in one group. A
set of six specific changes to the energies of the electrons covers all of
the logic gates required for quantum computing, according to the
researchers. Quantum programs would consist of timed sequences of the

The main disadvantage of the researchers' proposal is that it could
require as many as two spins per qubit rather than the usual single spin,
which would make for a larger device, said Benjamin. "Right now
experimentalists are struggling to make even two qubits in solid-state
systems," he said.

The researchers' work is valuable because it extends the range of
candidates for quantum computing, said Barry Sanders, a professor of quantum
information science at the University of Calgary in Canada. The work is
"stoking the fires of creativity so that we physicists can dream up other
quantum computing realizations that lead to easier control and less
experimental complexity," he said.

There is a growing realization that there are many ways to perform
qubit operations, said Robert Joynt, a physics professor at the University
of Wisconsin at Madison. The Oxford and University College London work is
significant for people trying to make a real machine, because it means that
the constraints on the hardware are a lot looser than people thought at
first, he said. This research "is particularly nice since it gets rid of the
usual need to precisely tune two-qubit operations."

The researchers are currently exploring how the method would work in a
two- or three-dimensional array of qubits, said Benjamin. "We'd also like to
build up a more detailed description of how to implement our scheme with
specific technologies like... electron spin," he said.

Researchers generally agree that practical quantum computers are two
decades away. It is possible that quantum computers capable of computations
that are impossible on conventional computers could be built within ten
years, said Benjamin.

Such systems "will be mainly of interest to the scientific community
because they will involve using quantum computers to simulate other quantum
systems, such as fundamental biological processes," said Benjamin. "These
first quantum computers may require an entire lab built around them, and may
be treated as a national or international resource for research -- a bit
like today's supercomputers or... particle accelerators."

However, it is also possible that quantum computing research could
stall if there's not enough experimental progress in the next few years,
said Benjamin. "It's possible that quantum computing is an idea born before
it's time. Our technology may simply be to crude to achieve it," he said.

Benjamin's research colleague was Sougato Bose. The work appeared in
the June 20, 2003 issue of Physical Review Letters. The research was funded
by the Royal Society, the Oxford-Cambridge-Hitachi Nanoelectronics at the
Quantum Edge project in England, and the National Science Foundation (NSF).

Timeline: 10-20 years
Funding: Corporate, Government, University
TRN Categories: Quantum Computing and Communications
Story Type: News
Related Elements: Technical paper, "Quantum Computing with an
Always-On Heisenberg Interaction," Physical Review Letters, June 20, 2003.

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