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Quantum Corrections
How to check errors in a quantum computer

CONTEXT: To an outsider, the logic of quantum computing can seem mystical. While a standard bit represents data as one way or another (digital 0 or 1), a quantum bit stores data as one way and another (0 and 1 and all possibilities in between). While a standard computer must crunch through possible solutions one at a time, a quantum computer could, in theory, survey all solutions at once and pick the correct one in a single step. This is ideal for solutions that rely on trial and error, such as breaking encryption codes.

But, like some cursed mythical creature, much of the information contained in a quantum system will vanish if it is observed, because the process of looking at it disturbs the system. That means a user can look at the answer to a question but can’t check the calculations behind it. A quantum computer therefore needs to correct errors reliably without anyone actually seeing them. Now, for the first time, John Chiaverini and colleagues from the National Institute of Standards and Technology (NIST) have done this in a quantum system that could be scaled up.

METHODS AND RESULTS: In the NIST quantum computer, information is encoded in a single atom’s quantum state. Using a process called entanglement, the fate of this “parent atom” is linked to that of two companion atoms, so that changes to the parent’s condition are reflected in the companions. Using beryllium ions (atoms with electric charge) to carry quantum information, the researchers were able to disentangle, decode, and compare the states of the two companion ions and thus indirectly deduce whether an error had occurred. A laser pulse could then correct the original ion’s quantum state without actually observing it.

WHY IT MATTERS: Many encryption techniques depend on the difficulty of factoring very large numbers through trial and error. A quantum computer could, in theory, defeat all such encryption systems and promises to be orders of magnitude more powerful than the most advanced systems today. So anyone interested in keeping digital secrets – from credit card numbers for Web transactions to classified information for governments and corporations – cares about quantum computing. Although a useful quantum computer is still far, far away, the work at NIST has shown how to lift one of the most bedeviling curses of quantum mechanics.

SOURCE: Chiaverini, J., et al. 2004. Realization of quantum error correction. Nature 432:602-605.

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