Has Google really unlocked quantum supremacy? Not so fast, says IBM

Last month, a leaked research paper from Google suggested a significant quantum breakthrough. IBM's research team now suggests the claim was a little premature.

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Last month, a research paper briefly posted on a Nasa website – and since removed – suggested that Google had achieved quantum supremacy. This means that its quantum computer could, in theory, take on calculations that are impossible even for the world's highest-performing supercomputers.

The paper claimed that Google's 53-qubit quantum system had taken three minutes and 20 seconds to carry out a calculation that would have taken the world's most advanced classical computers 10,000 years to complete.

But now, IBM's research shows that a classical system can actually perform the same task in two and a half days, "and with far greater fidelity". What's more: according to the researchers, that is a worst-case scenario estimate.

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That's hardly a case for quantum supremacy. Where Google went wrong, said IBM's staff in a blog post, was in underestimating the resources of classical computers, and particularly their data storage capacity.

"When (Google's) comparison to classical was made," they said, "they (...) failed to account for plentiful disk storage." IBM, therefore, proceeded to simulate the same task using both primary and secondary storage to extend the reach of supercomputers.

In addition to a hierarchy of memories, IBM pointed to a palette of performance-enhancing methods that can further reduce the amount of disk space required to carry out the task, and in turn the time it takes to complete. "It is important to leverage all such capabilities when comparing quantum to classical," said its research team.

With all of this, IBM's team found that its simulation of a 53-qubit circuit required 64PB of disk space, and for 54-qubit circuits, 128PB were needed. All of which fall within the 250PB capacity of a supercomputer.

"Google's experiment is (...) showing state-of-the-art gate fidelities on a 53 qubit-device," concluded the researchers, "but it should not be viewed as proof that quantum computers are 'supreme' over classical computers."

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They further urged the public to take any news of "quantum supremacy" with a pinch of salt, insisting that quantum computers will work in concert with classical computers in the future rather than reign supreme over them. 

Quantum computing revenue is expected to increase from $1.9 billion in 2023 to $8 billion in 2027, and it will have myriad applications in physics, chemistry, AI and energy, among many others. But it could still be a while before quantum devices can outpace classical computers.