Quantum Leap

Imagine a computer with speed previously unheard of, the agility to solve calculations that would stymie a conventional computer and the ability to produce more secure lines of communication.

Impossible in the foreseeable future? Actually, researchers think this type of machine, in the form of a quantum computer, may move out of their laboratories by the end of the decade.

The largest quantum computer built to date, however, is a 5-bit device created by IBM's Almaden Research Center that has been squeezed onto a single molecule.

The concept of using quantum physics to perform simultaneous computations on massive amounts of information has been in the works since the 1980s. It was only about five years ago, however, that quantum computing became a major area of interest to computer theorists around the world.

Unlike a conventional computer, a quantum computer could conceivably process every possible input simultaneously through a series of quantum switches connected in parallel. In effect, it would be the most perfect form of parallel processing imaginable, dealing with information in a way that's impossible for conventional computers, which follow the rules of binary logic - an either/or distinction.

In a binary system, each bit of information is either on or off, one or zero, true or false. A conventional computer strings together combinations of ones and zeros to represent pieces of information, whereas quantum computers are made up of quantum particles such as electrons and atomic nuclei. Each particle represents a quantum bit, or qubit.

Qubits differ from conventional bits in that an atom or nucleus can be in a state of "superposition," acting as both a one and a zero simultaneously. Quantum computers take advantage of the behavior and properties of atoms to potentially provide switching and processing speeds millions of times faster than those of today's supercomputers.

For example, if you have two qubits, they could simultaneously exist as a combination of all possible two-bit numbers: 00, 01, 10 and 11. Add a third qubit, and you could have a combination of all possible three-bit numbers: 000, 001, 010, 011, etc. This system scales exponentially: n qubits can stand for 2n numbers at once. Line up a mere 50 qubits, and you could represent every binary number from zero to more than a trillion - simultaneously.

Overcoming Obstacles

A major obstacle that researchers face in developing quantum computers is making sure the qubits retain their superposition of being both - or either - a one or a zero. Observation of a quantum computer allows an outside interference such as light or noise to exert some influence over the qubits, forcing them to collapse and leaving an ordinary computer based on ones and zeros.

In order to allow quantum states to store information, a quantum computer can't interact with its environment. But at the same time, it has to be manipulated to allow calculations to be performed.

"Reliability is a serious factor for quantum computers. We need to devise ways of coding information so that qubits will not be affected by the environment - a fault-tolerant effect," says John Preskill, a professor of theoretical physics at the California Institute of Technology in Pasadena, Calif.

Quantum computers hold great promise in the area of cryptography. The transmission of encrypted data over fiber-optic communications using single photons (packets of light) could be used to foil code breakers. Qubits can't be copied or cloned, so it would be virtually impossible for a hacker to break code encrypted with a quantum computer.

On the other hand, "if a hacker possessed a quantum computer . . . security would be threatened because he or she would be able to break the codes of conventional computers," says Preskill.

Carl Williams, a physicist at the National Institute of Standards and Technology in Gaithersburg, Md., and Umesh Vazirani, a professor of computer science at the University of California, Berkeley, say absolutely safe lines of communication will require a quantum repeater.

A quantum repeater would allow the photon being transmitted over optical fiber to be repeated without being disturbed. The repeater would allow the photon to be transmitted another 50 km. For this to work, the photon would need to be duplicated in its superposition (the pair of complex numbers describing its position).

"A quantum repeater is possible in principle, and scientists are trying to make it happen. The main challenge is that the repeater must be a quantum device - some kind of quantum switch, since any measurement of the photon would reveal only a very small amount of information about the quantum state of the photon," says Vazirani.

Scientists need time to work through the challenges of quantum computing, including the development of a computer with capacity of 50 qubits or more, but the important first steps toward that goal are currently under way, according to Vazirani.

Copyright © 2000 IDG Communications, Inc.

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