Quantum demo does tricky computing

By Eric Smalley, Technology Research News

Quantum computers can theoretically solve problems that are beyond even the most powerful possible classical computer -- like cracking secret codes -- by using the bizarre properties of quantum particles to search through large numbers of possible answers at once.

Scientists from IBM Research and Stanford University have built a quantum computer out of seven atoms and used the computer to show that factoring the number 15 results in the numbers 3 and 5.

Though seven atoms doesn't sound like a lot and factoring 15 is not a big problem, the device is something of a milestone in quantum computing. Seven atoms constitute a large device by the standards of the prototype quantum computers built to date, and running a factoring algorithm on the atoms shows that they can be controlled well enough to process information.

The researchers' device is unlikely to lead directly to a practical quantum computer, but their results could make it easier to design and build quantum computers in general. "Showing that we can factor 15 with a quantum computer is akin to how researchers demonstrated early electronic computers calculating digits of the number Pi," said Isaac L. Chuang, now an associate professor at the Massachusetts Institute of Technology. "It is a milestone, but not a useful feat in and of itself."

The researchers' quantum computer consisted of five fluorine and two carbon atoms that were part of a molecule suspended in a test tube of liquid. Particles like atoms and electrons spin either up or down, similar to a top spinning clockwise or counterclockwise, and these spin directions can represent the ones and zeros of computing.

The researchers turned these atomic quantum bits on and off with a series of carefully timed radio wave pulses that reversed the spins of the atoms. This nuclear magnetic resonance (NMR) quantum computing method is based on the same technology used in MRI medical imaging machines.

What makes quantum bits, or qubits, more powerful than regular computer bits is that when quantum particles are isolated from the environment and cannot be observed, they enter the quantum mechanical state of superposition, which means they are in some mixture of both spin up and spin down. This allows a qubit to represent both one and zero at the same time, and a relatively small number of qubits to represent many numbers at once.

Particles can also be linked, or entangled. When changes are made to one entangled particle, they all change the same way regardless of the physical distance between them, as long as they remain in superposition. Using this bizarre property, quantum computers can theoretically examine every possible answer to a problem with one series of operations rather than having to check each individually, which means they could solve problems that are beyond the capabilities of the most powerful classical computer conceivable.

The way the researchers simulated, designed and operated their computer is probably more significant than what they did with it. "[That] we know how to accurately model errors occurring to large-scale, complicated quantum information processing systems will be the most useful technical component of our achievement," said Chuang.

Researchers generally agree that liquid nuclear magnetic resonance is unlikely to lead to practical quantum computers because it is probably not possible to make NMR quantum computers much bigger than seven qubits. However, the way the researchers use the spin of the atoms to compute is compatible with many quantum computer designs, including those based on semiconductor devices. "The methods we demonstrated for controlling these spins... will generally be how future quantum information processing machines are controlled and programmed," said Chuang.

The research "is an exquisite demonstration of control over complex pulse sequences combined with a growing bag of tricks for compiling quantum computing circuits," said Daniel Lidar, an assistant professor of chemistry at the University of Toronto. "There is no doubt that these techniques... will be useful for eventual scalable solid-state quantum computing implementations."

The researchers' experiment is one of only a small number that have implemented such complex algorithms, said Emanuel Knill, a mathematician at Los Alamos National Laboratory. "The real significance is in the demonstration of techniques for the control of quantum computers. Any other comparably complex algorithm with a definite and verifiable answer can serve this purpose," he said.

Unfortunately, the researchers did not provide the scales necessary to compare their data, said Knill. "This makes it impossible to determine how well their experiment worked and how well the measured [results] compared to simulation. As a result, the value of this contribution as a demonstration of quantum control is significantly lessened," he said.

According to many researchers, it is likely to be at least 20 years before practical quantum computers can be built.

There is also a chance that practical general-purpose quantum computers will never be built, said Chuang. "Classical computing itself is growing in performance in leaps and bounds, and in terms of raw computational power, quantum computers may never be competitive," said Chuang.

Chuang's research colleagues were Lieven M. K. Vandersypen and Mathias Steffen of Stanford University and IBM Research, and Gregory Breyta, Costantino S. Yannoni and Mark H. Sherwood of IBM Research. They published the research in the December 20/27, 2001 issue of the journal Nature. The research was funded by IBM and the Defense Advanced Research Projects Agency (DARPA).

Timeline:   20 years
Funding:   Corporate; Government
TRN Categories:   Quantum Computing
Story Type:   News
Related Elements:  Technical paper, "Experimental realization of Shor's quantum factoring algorithm using nuclear magnetic resonance," Nature, December 20/27, 2001


January 2, 2002

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