September 2005

R&D 2005

Continued from page 3

By Technology Review

Bell Labs
Quantum Computing
By Dan Cho

More than half a century after inventing the transistor--the foundation for modern electronics, computing, and telecommunications--Lucent Technologies' Bell Labs is pursuing another technology that could radically change information technology: quantum computing. Today's transistors continue to get smaller, allowing computer speeds to double every one or two years. But a quantum computer would leap way ahead of that pace. If such a machine is finally built, it will offer the ability to solve certain 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 defies intuition--some quantum blend of those two values. Another quantum effect known as "entanglement" allows two or more qubits to coordinate their behavior, even when they don"t appear to be interacting.

These strange properties would make qubits extremely powerful tools for attacking certain computing problems, such as factoring large prime numbers in encryption 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 particles must be isolated so that they can"t interact with their surrounding environment, which would ruin 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 electrodes that can hold an ion in place above it in an electric field, while a laser beam alters the ion's spin. When the computation is complete, the ion is excited by a different laser, causing it to give off photons that can be recorded by a camera to reveal its final state, which represents part of the answer to a problem.

Research groups working with trapped ions have so far produced quantum computations 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 different regions to prepare them for a computation, read their final states, and even store them in memory.

While most ion traps are currently made of ceramic, Bell Labs is working to design a multiplex system in silicon. Transistors could supply voltage from an external source wherever it"s needed, eventually allowing researchers 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 quantum computing research. According to Wineland, the ceramic traps that scientists have been using in current experiments have "obvious limits." But what will ultimately replace them, he says, "is still open for question."

Building ion traps on silicon would allow 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 stoppers," says Bishop.

Still, most researchers in the field, including Wineland and Slusher, do not expect a practical quantum computer to appear for at least another decade. Even then, the first machines will be built to solve very specific computing tasks. And while solving just, say, the factoring problem would have profound implications in cryptography, a quantum computer may not be any better than a conventional machine for many of the tasks that a desktop PC routinely handles.

None of this dissuades Bell Labs--which has eliminated much of its fundamental R&D in recent years--from pursuing what is, really, still a basic research project. Part of its motivation is the belief that the hardware research may pay off for Lucent long before quantum computers arrive, yielding advances in areas such as miniaturized lasers and optical components. "What we learn from working in the quantum computing field may someday lead to commercialization," says Bishop, "but more importantly, it also drives discoveries that could improve today"s communications and computing technology."