Quantum computer used to solve problems

Quantum computers get their power by taking advantage of certain quantum physics properties of atoms or nuclei that allow them to work together as quantum bits, or "qubits," to be the computer's processor and memory. By interacting with each other while being isolated from the external environment, theorists have predicted -- and this new result confirms -- that qubits could perform certain calculations exponentially faster than conventional computers.

The new quantum computer contains five qubits -- five fluorine atoms within a molecule specially designed so the fluorine nuclei's "spins" can interact with each other as qubits, be programmed by radiofrequency pulses and be detected by nuclear magnetic resonance instruments similar to those commonly used in hospitals and chemistry labs.

Using the molecule, Chuang's team solved in one step a mathematical problem for which conventional computers require repeated cycles. The problem is called "order-finding" -- finding the period of a particular function -- which is typical of many basic mathematical problems that underlie important applications, such as cryptography.

In particular, the order-finding problem can be described by considering a large number of rooms and an equal number of randomly placed one-way passages, some of which may loop back upon themselves into the same room. It is certain that at some point, a person moving through the rooms and passages will return to the starting room. The problem is to calculate with the least number of queries, the minimum number of passages through which one must travel through before returning to the starting room. IBM Research's 5-qubit quantum computer solved any case of the problem in one step, while a conventional approach would require up to four steps, depending on the particular case. The new result confirmed the predictions of Prof. Richard Cleve of the University of Calgary in Canada made earlier this year.

While the potential for quantum computing is huge and recent progress is encouraging, the challenges remain daunting. IBM's 5-qubit quantum computer is a research instrument. Commercial quantum computers are still many years away, since they must have at least several dozen qubits before difficult real-world problems can be solved.

"This result gives us a great deal of confidence in understanding how quantum computing can evolve into a future technology," Chuang says. "It reinforces the growing realization that quantum computers may someday be able to live up to their potential of solving in remarkably short times problems that are so complex that the most powerful supercomputers can't calculate the answers even if they worked on them for millions of years."

Chuang says the first applications are likely to be as a co-processor for specific functions, such as database lookup and finding the solution to a difficult mathematical problem. Accelerating word processing or Web surfing would not be well-suited to a quantum computer's capabilities.

Chuang will present his team's latest result today at Stanford University at the Hot Chips 2000 conference, which is organized by the Institute of Electrical and Electronics Engineers' (IEEE) Computer Society. His co-authors are Gregory Breyta and Costantino S. Yannoni of IBM-Almaden, Stanford University graduate students Lieven M.K Vandersypen and Matthias Steffen, and theoretical computer scientist Richard Cleve of the University of Calgary. The team has also submitted a technical report of their experiment to the scientific journal, Physical Review Letters.