Sight unseen: quantum errors found, fixed.

Bits of information in a quantum computer suffer constant peril. Because mere atoms are the keepers of data in the rudimentary machines built so far, small amounts of random energy can easily change bits, sowing errors that are deadly to reliable computing.

Scientists have proposed various methods to repair such mistakes, but a team of scientists has now turned theory into practice by successfully demonstrating a specific quantum method to correct one type of error. The technique might never be practical itself, but it improves the outlook for useful quantum computers. Theorists predict that quantum computers can outperform conventional computers for certain types of calculations (SN: 6/14/97, p. 367).

Quantum computers derive their unique power, but also their fragility, from the rules of quantum mechanics. Quantum bits, or qubits, can represent not just 0 or 1, as in conventional computers, but also mixtures of 0 and 1.

In the past few years, theorists have proposed that quantum computers could surmount their error-prone nature by continually fixing errors as they crop up (SN: 2/21/98, p. 127). Toward that goal, they devised procedures for using numerous copies of a bit to both flag errors and indicate the correct settings. Although these extra bits contain the key to repairing errors, quantum mechanics won't allow the mingled states of the bits to be directly measured. Reading a qubit causes it to collapse into just one state, wiping out data, so theorists have proposed ways to indirectly observe a qubit's status.

Experiments described in the Sept. 7 Physical Review Letters offer the first laboratory evidence that such quantum error-correcting schemes work in practice. A research team led by David G. Cory at the Massachusetts Institute of Technology used the simplest form of quantum error correction--a three-bit code--to find and fix errors in the qubit blend of Os and 1s. Scientists from Harvard University, Los Alamos (N.M.) National Laboratory, and Bruker Instruments in Billerica, Mass., also participated in the experiments.

The group used strong magnetic fields and radio wave pulses to manipulate the nuclear spins of atoms in pencil-size test tubes of liquids, such as a solution of the amino acid alanine. The technique, called nuclear magnetic resonance (SN: 1/18/97, p. 37), is commonly used in medicine to reveal soft tissue structure. To avoid the pitfall of directly measuring qubits, the team gingerly probed for differences among the three error-correction qubits, which are identical in the absence of errors.

The researchers expect to achieve "full-blown" error correction in the next few months using 5-bit codes that also deal with a second type of bit error, says Raymond Laflamme, a Los Alamos member of the team. However, he warns, the error-correction scheme verified by the team ultimately falls short as a practical approach because it excessively weakens radio signals transmitted by the atomic nuclei qubits so it can't be used to continuously monitor for errors.

Nonetheless, "it's an important demonstration of the concept," says David P. Di-Vincenzo of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y.