Overcoming quantum error.

In principle, quantum computers can solve certain types of mathematical problems in dramatically fewer steps than conventional computers (SN: 1/14/95, p. 30). Quantum information is fragile and easily disturbed, however, so handling combinations of quantum states, such as electron spins, presents considerable difficulties. The entire system must be well isolated from the environment, yet the quantum components need to interact strongly to perform a computation.

Although the recent discovery of ways to correct errors in the values of bits in a quantum computer provided an encouraging sign that quantum computation could eventually be made practical (SN: 1/20/96, p. 38), the error-correcting codes worked best when the faults occurred randomly and independently. In the Jan. 16 Science, Emanuel Knill, Raymond Laflamme, and Wojciech H. Zurek of the Los Alamos (N.M.) National Laboratory show how it is possible in theory to reduce an entire system's error rate to acceptable levels, even when the faults arise from nonrandom interactions between quantum bits.

The strategy developed by Zurek and his coworkers for what they call "resilient quantum computation" combines quantum error correction, which preserves the values of quantum bits in memory, with methods of ensuring that bits are encoded correctly and that logic operations are completed properly. The researchers show that, as long as the error rate per operation remains below a threshold value, an accurate computation can result.

"The threshold results demonstrate that quantum computation is possible in the presence of physically reasonable sources of noise," Zurek and his coworkers conclude. "Whether resilient quantum computation can be implemented in practice remains to be seen."