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Quantum computers are largely theoretical devices that could perform some computations exponentially faster than conventional computers can. Crucial to most designs for quantum computers is quantum error correction, which helps preserve the fragile quantum states on which quantum computation depends.

The ideal quantum code would correct any errors in quantum data, and it would require measurement of only a few quantum bits, or , at a time. But until now, codes that could make do with limited measurements could correct only a limited number of errors—one roughly equal to the square root of the total number of qubits. So they could correct eight errors in a 64-qubit quantum computer, for instance, but not 10.

In a paper they're presenting at the Association for Computing Machinery's Symposium on Theory of Computing in June, researchers from MIT, Google, the University of Sydney, and Cornell University present a new code that can correct errors afflicting a specified fraction of a computer's qubits, not just the square root of their number. And that fraction can be arbitrarily large, although the larger it is, the more qubits the computer requires.

"There were many, many different proposals, all of which seemed to get stuck at this square-root point," says Aram Harrow, an assistant professor of physics at MIT, who led the research. "So going above that is one of the reasons we're excited about this work."

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