Google has claimed a breakthrough in correcting for the errors that are inherent in today’s quantum computers, marking an early but potentially significant step in overcoming the biggest technical barrier to a revolutionary new form of computing.
The internet company’s findings, which have been published in the journal Nature, mark a “milestone on our journey to build a useful quantum computer”, said Hartmut Neven, head of Google’s quantum efforts. He called error correction “a necessary rite of passage that any quantum computing technology has to go through”.
Quantum computers struggle to produce useful results because the quantum bits, or qubits, they are based on only hold their quantum states for a tiny fraction of a second. That means information encoded in a quantum system is lost before the machine can complete its calculations. Finding a way to correct for the errors this causes is the hardest technical challenge the industry faces.
Some quantum start-ups have pinned their hopes in the near term on finding ways to program today’s error-prone, or “noisy”, machines, even if this only leads to a small improvement over traditional computers. However, those efforts have yet to yield practical results, leading to a growing view that quantum computing will not be useful until the far bigger problem of error correction has been solved.
Google’s researchers said they had found a way to spread the information being processed in a quantum computer across a number of qubits in a way that meant the system as a whole could retain enough to complete a calculation, even as individual qubits fell out of their quantum states.
The research published in Nature pointed to a reduction of only 4 per cent in the error rate as Google scaled up its technique to run on a larger quantum system. However, the researchers said this was the first time that increasing the size of the computer had not also led to a rise in the error rate. Neven said that it showed Google had passed a “break-even point” after which further advances would bring steady gains in performance, putting the company on a path to having its first practical quantum computer.
The breakthrough in error correction was the result of improvements Google had made to all components of its quantum computer, from the quality of its qubits to its control software and the cryogenic equipment used to chill the computer to near-absolute zero, according to Julian Kelly, a Google researcher. This had reduced the number of errors to a low enough point that the size of the system could be increased without leading to an exponential rise in the error rate, he added.
Google described the breakthrough as only the second of six steps it needed to go through to build a practical quantum computer. The next step involved perfecting its engineering so that it would need only 1,000 qubits to create a so-called logical qubit — an abstraction, built on top of the imperfect physical qubits, that can function without errors. Neven said Google believed it would have a useful machine once it had worked out how to build and link 1,000 logical qubits into a single system.
Google’s research claims around quantum computing have proved controversial in the past. In 2019, it claimed in an article in Nature that it had achieved so-called quantum supremacy — the point at which a quantum computer can complete a calculation that is for all intents and purposes impossible for a traditional machine.
However, that assertion was challenged by IBM and others, and new programming techniques were developed to increase the performance of traditional computers, pushing back the moment when the makers of quantum machines could claim to have reached “supremacy”.
In this week’s Nature paper, Google’s researchers said they were taking a “cautious” stance on their latest claimed breakthrough. They warned there was still a small chance that their error-correction technique would not work when applied to much larger quantum systems in future.