Home' Technology Review : September 2005 Contents 55
ahead of that pace. If such a machine is nally built, it will o er the ability to solve cer-
tain problems millions of times faster.
A conventional computer stores information as bits, which are represented as 1s and
0s. Quantum computers rely on quantum bits, or qubits, which can hold values of 1, 0,
or---and this is the part that de es intuition---some quantum blend of those two values.
Another quantum e ect known as "entanglement" allows two or more qubits to coördi-
nate their behavior, even when they don't appear to be interacting.
These strange properties would make qubits extremely powerful tools for attack-
ing certain computing problems, such as factoring large prime numbers in encryp-
tion applications and searching huge databases. (Two Bell Labs researchers, Peter
Shor and Lov Grover, devised breakthrough quantum algorithms for solving these
two problems in the 1990s.)
But creating the hardware that can harness qubits presents a huge challenge. Qubits
are encoded as the spins of individual particles like atoms, ions, or photons. These par-
ticles must be isolated so that they can't interact with their surrounding environment,
which would r uin the quantum computation. Bell Labs researchers, like several other
groups, are pursuing a method for controlling qubits with a device called an ion trap.
Each trap is between a tenth and a hundredth of a millimeter long and has tiny elec-
trodes that can hold an ion in place above it in an electric eld, while a laser beam alters
the ion's spin. When the computation is complete, the ion is excited by a di erent laser,
causing it to give o photons that can be recorded by a camera to reveal its nal state,
which represents part of the answer to a problem.
Research groups working with trapped ions have so far produced quantum computa-
tions using fewer than 10 qubits. To be of any practical use, though, a quantum computer
will require hundreds or thousands of qubits. The qubits might be held in an array of
many traps, known as a multiplex system, with connections for shuttling ions back and
forth between di erent regions to prepare them for a computation, read their nal
states, and even store them in memory. While most ion traps are currently made of ce-
ramic, Bell Labs is working to design a multiplex system in silicon. Transistors could
supply voltage from an exter nal source wherever it's needed, eventually allowing re-
searchers to position thousands of ion traps on a single chip, says Richart Slusher, head
of Bell Labs' quantum computing team. Bell Labs expects to fabricate some of these
multiplex traps in the next two years, says Slusher.
The Bell Labs group has "thought about the long-range problem, including how
you do all the electronic controls," says David Wineland, head of the Ion Storage
group at the National Institute of Standards and Technology, a leading center of quan-
tum computing research. According to
Wineland, the ceramic traps that scien-
tists have been using in current experi-
ments have "obvious limits." But what
will ultimately replace them, he says, "is
still open for question."
Building ion traps on silicon would al-
low researchers to take advantage of the
semiconductor industry's decades of
working knowledge. David Bishop, Bell
Labs' vice president for physical-sciences
research, thus believes that all the basic
technologies for quantum computing are
ready---or that they soon will be. "We
don't see any fundamental show stop-
pers," says Bishop.
Still, most researchers in the eld, in-
cluding Wineland and Slusher, do not ex-
pect a practical quantum computer to
appear for at least another decade. Even
then, the rst machines will be built to
solve very speci c computing tasks. And
while solving just, say, the factoring prob-
lem would have profound implications in
cryptography, a quantum computer may
not be any better than a conventional ma-
chine for many of the tasks that a desktop
PC routinely handles.
None of this dissuades Bell Labs---
which has eliminated much of its fun-
damental R&D in recent years---from
pursuing what is, really, still a basic re-
search project. Part of its motivation is
the belief that the hardware research may
pay o for Lucent long before quantum
computers arrive, yielding advances in
areas such as miniaturized lasers and op-
tical components. "What we learn from
working in the quantum computing eld
may someday lead to commercializa-
tion," says Bishop, "but more impor-
tantly, it also drives discoveries that could
improve today's communications and
A computer that manipulated
quantum qubits could solve certain
problems millions of times faster
than today's machines.
Fluorescing ions trapped in an electric field
could calculate the answers to computing
problems that are impossibly large today.
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