computers go digital
Technology Research News
Some of the same properties that would
make quantum computers phenomenally powerful are also properties that
make it difficult to actually build them.
Problems that would take the fastest possible classical computer longer
than the lifetime of the universe to solve would be hours-long exercises
for large-scale quantum computers. Such machines would be able to rapidly
search huge databases and would render today's encryption methods useless.
The key to quantum computers' potential is that quantum bits, the basic
building blocks of quantum computing logic circuits, can represent a mix
of 1 and 0 at the same time, allowing a string of qubits to represent
every possible answer to a problem at the same time. This means a quantum
computer could check every possible answer using a single set of operations.
Classical computers, in contrast, check each answer one at a time.
But today's qubits are difficult to work with and prone to errors, and
the faster they go the more errors they produce. One of the challenges
of building a quantum computer is reducing errors. Researchers from the
University of Wisconsin at Madison have eased the problem with a method
that reduces error rates by two orders of magnitude.
Today's computers are digital, meaning they use signals that are either
on or off to represent two states -- a 1 or a 0 -- and all computations
are done using combinations of these binary numbers. One advantage of
using just two states is the signals that represent those states don't
have to be exact, they simply have to be clearly closer to 1 than 0 or
Qubits are analog devices, meaning they produce variable, continuous signals
rather than discrete on and off states. For example, a particle can be
in one of two orientations, spin up and spin down, but also some mix of
the two. The 1s and 0s of digital information are mapped to the spin up
and spin down states, but quantum computations have to be precise to ensure
that the given particle is actually in one of those two states. "Classical
bits have only two states... quantum bits can be in between," said Robert
Joynt, a physics professor at the University of Wisconsin at Madison.
A qubit continually rotates between 0 and 1, which makes it prone to errors,
said Joynt. "A rotation of a qubit can, for example, fall a little bit
short with only a very minor error in the input signal," he said.
The researchers' method makes quantum computing a pseudo-digital operation.
"In our set-up, a definite rotation rate for the qubits is associated
with a range of input signals. [This way] the input does not have to be
exceedingly precise," said Joynt.
Easing the requirements for precision could go a long way toward making
quantum computers viable. "The driving force [for the idea] was objections
from experienced electrical engineers, particularly at IBM, who believed
that quantum computing would not work... the because the specs for the
driving electronics would be much too [demanding]," said Joynt.
The researchers are applying the pseudo-digital qubits to their ongoing
efforts to build a solid-state quantum computer. Their design calls for
thousands of individually-controlled electrons in a silicon chip. The
chip would allow for careful control of the interactions between neighboring
electrons so that the states of the electrons could be used to carry out
computations. Some of the fundamental logic operations in quantum computers
are carried out through the interactions of pairs of qubits.
The researchers added the pseudo-digital qubits concept to their design
by having pairs of electrons slide past each other rather than crash into
each other, said Joynt. When the electrons are well separated the interaction
is off, representing a 0, and when they are within close range the interaction
is on, representing a 1.
When the researchers simulated the technique, they found that it reduced
operational error rates by more than two orders of magnitude, according
to Joynt. The researchers' pseudo-digital qubits could be implemented
in other types of quantum computers, he added.
The pseudo-digital approach is a good one, said Bruce Kane, a visiting
associate research scientist at the University of Maryland. "My guess
is that future quantum computers will use the pseudo-digital approach,"
he said. It remains to be seen whether the devices the researchers are
building will work well, however, he said.
Quantum computing naturally has many similarities to analog rather than
digital computing, said Kane. Because digital computers operate using
just two states -- 1 and 0 -- inputs can always be rounded. This type
of rounding, however, is impossible in quantum computing, he said. "It
[is usually] necessary to control parameters very precisely to keep the
computation on track," he said.
The researchers' method is an attempt to find systems that "pretty much
automatically have only two interaction strengths," said Kane. No system
can have exactly this behavior, so the method doesn't eliminate the problem
of errors creeping into a quantum computation, but it can reduce the severity
of the errors, he said.
The researchers have shown how to minimize the adverse effects of turning
interactions on and off in quantum computing, said Seth Lloyd, a professor
of mechanical engineering at the Massachusetts Institute of Technology.
"Although I doubt that this exact architecture will prove to be the one
that is used to construct large-scale quantum computers, it is exactly
this sort of imaginative quantum-mechanical engineering that is required
to solve the problems of large-scale quantum computation," he said.
One of the challenges in implementing the scheme in a real quantum computer
is fabricating the tiny qubits precisely, said Joynt. "The real issue
is fabrication of quite complicated nanostructures," he said.
The researchers are working on qubits made from two basic pieces -- a
semiconductor sandwich structure "which is really a monster club sandwich,"
said Joynt; and a gate structure, which controls the state of a qubit
so that it can represent a one or a zero.
The researchers have made progress on the semiconductor sandwich structure
and are gearing up now to produce the gate structure, "which is quite
complex," Joynt said.
The researchers are also working on a readout apparatus that will fit
on the chip. Reading the quantum states of particles is tricky because
quantum states are easily disturbed.
It will take a decade to develop simple demonstration models, and probably
20 years before the devices can be used in practical quantum computers,
Joynt's research colleagues were Mark Friesen and M. A. Eriksson. They
published the research in the December 9, 2002 issue of Applied Physics
Letters. The research was funded by the National Science Foundation (NSF)
and the Army research office (ARO).
Timeline: 10-20 years
TRN Categories: Physics; Quantum Computing and Communications
Story Type: News
Related Elements: Technical paper, "Pseudo-Digital Quantum
Bits," Applied Physics Letters, December 9, 2002.
29/February 5, 2003
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