Battling Decoherence: the Fault-tolerant Quantum Computer

THE FAULT-TOLERANT QUANTUM COMPUTER I carried by a quantum system has notoriously weird properties. Physicists and engineers are now learning how to put that weirdness to work. Quantum computers, which manipulate quantum states rather than classical bits, may someday be able to perform tasks that would be inconceivable with conventional digital technology. (See the article by Charles H. Bennett, PHYSICS TODAY, October 1995, page 24, and the "Search and Discovery" report in PHYSICS TODAY, March 1996, page 21.) Formidable obstacles must be overcome before largescale quantum computers can become a reality (see the article by Serge Haroche and Jean-Michel Raimond, PHYSICS TODAY, August 1996, page 51). A particularly daunting difficulty is that quantum computers are highly susceptible to making errors. The magical power of the quantum computer comes from its ability to process coherent quantum states; but such states are very easily damaged by uncontrolled interactions with the environment—a process called decoherence. In response to the challenge posed by decoherence, the new discipline of quantum error correction has arisen at the interface of physics and computer science. We have learned that quantum states can be cleverly encoded so that the debilitating effects of decoherence, if not too severe, can be resisted.