Quantum Advantage Showdowns have no clear winners

Last month, physicists at Toronto-based startup Xanadu published a curious experiment in Nature in which they generated seemingly random numbers. During the pandemic, they built a desktop machine called Borealis, which consisted of lasers, mirrors, and over a kilometer of fiber optics. Inside Borealis, 216 beams of infrared light bounced through an intricate network of prisms. Then a bank of detectors counted the number of photons in each beam after they traversed the prisms. Ultimately, the machine-generated 216 numbers at once, corresponding to the number of photons in each respective beam.

Borealis is a quantum computer, and according to Xanadu researchers, this laser-powered rolling dice exceeds the capabilities of classical or non-quantum-based computers. It took Borealis 36 microseconds to generate a set of 216 numbers from a complicated statistical distribution. They estimated that it would take Fugaku, the most powerful supercomputer at the experiment, an average of 9,000 years to produce a series of numbers from the same distribution.

The experiment is the latest in a series of demonstrations of the so-called quantum advantage, in which a quantum computer beats a cutting-edge supercomputer on a specific task. The experiment “pushes the limits of the machines we can build,” says physicist Nicolas Quesada, a member of the Xanadu team now working at Polytechnique Montréal.

“This is a major technological advance,” says Laura García-Álvarez of Chalmers University of Technology in Sweden, who was not involved in the experiment. “This device performed a calculation considered difficult for classical computers. But it doesn’t mean useful commercial quantum computing.”

So what exactly does Xanadu’s claim of quantum advantage mean? Caltech physicist John Preskill coined the concept in 2011 as “quantum supremacy,” which he has described as “the point at which quantum computers can do things that classical computers can’t, regardless of whether those tasks are useful.” (Since then, many researchers in the field have come to call it “quantum advantage” to avoid echoes of “white supremacy” as a useful task — which hasn’t been the case.)

Preskill’s words suggested that achieving a quantum advantage would mark a turning point and mark the beginning of a new technological era, where physicists would begin to design useful tasks for quantum computers. In fact, people were so eagerly awaiting the milestone that the first claim of a quantum computer outperforming a classical computer — by Google researchers in 2019 — was leaked.

But as more researchers claim the quantum advantage for their machines, the meaning of this achievement becomes murkier. For one thing, the quantum advantage does not mark the end of a race between quantum and classical computers. It’s the beginning.

Any claim of quantum advantage has prompted other researchers to develop faster classical algorithms to challenge that claim. In the case of Google, the researchers conducted a random number generation experiment similar to that of Xanadu. They wrote that it would take a state-of-the-art supercomputer 10,000 years to generate a collection of numbers, while their quantum computer took just 200 seconds. A month later, IBM researchers argued that Google had used the wrong classical algorithm to compare and that a supercomputer should only take 2.5 days. In 2021, a team using the Sunway TaihuLight supercomputer in China showed it could complete the task in 304 seconds – just a hair slower than Google’s quantum computer. An even larger supercomputer could run the algorithm in dozens of seconds, says physicist Pan Zhang of the Chinese Academy of Sciences. That would put the classic computer back on top.

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