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Google has shown that its Sycamore quantum computer can detect and fix computational errors, an essential step for large-scale quantum computing, but its current system generates more errors than it solves.

Error-correction is a standard feature for ordinary, or classical, computers, which store data using bits with two possible states: 0 and 1. Transmitting data with extra “parity bits” that warn if a 0 has flipped to 1, or vice versa, means such errors can be found and fixed.

In quantum computing the problem is far more complex as each quantum bit, or qubit, exists in a mixed state of 0 and 1, and any attempt to measure them directly destroys the data. One longstanding theoretical solution to this has been to cluster many physical qubits into a single “logical qubit”. Although such logical qubits have been created previously, they hadn’t been used for error correction until now.

Julian Kelly at Google AI Quantum and his colleagues have demonstrated the concept on Google’s Sycamore quantum computer, with logical qubits ranging in size from five to 21 physical qubits, and found that logical qubit error rates dropped exponentially for each additional physical qubit. The team was able to make careful measurements of the extra qubits that didn’t collapse their state but, when taken collectively, still gave enough information to deduce whether errors had occurred.

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