Quantum Advantage matchups have no clear winners

Last month, physicists at Toronto startup Xanadu published a curious experiment in Nature in which they generated seemingly random numbers. During the pandemic, they built a tabletop machine named Borealis, made up of lasers, mirrors, and over a mile of fiber optics. In Borealis, 216 beams of infrared light bounced through an intricate array of prisms. Then a series of detectors counted the number of photons in each beam after passing through the prisms. In the end, the machine generated 216 numbers at once – a number corresponding to the number of photons in each respective beam.

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

The experiment is the latest in a series of demonstrations of what is known as the quantum advantage, where a quantum computer beats a cutting-edge supercomputer at a specified task. The experiment “pushes the limits of the machines we can build,” says physicist Nicolas Quesada, a member of the Xanadu team who now works at Polytechnique Montreal.

“It’s a big technological breakthrough,” says Laura García-Álvarez from Chalmers University of Technology in Sweden, who was not involved in the experiment. “This device performed a calculation that is believed to be difficult for conventional computers. But that doesn’t mean useful commercial quantum computing.

So what exactly does Xanadu’s assertion of a quantum advantage mean? Caltech physicist John Preskill coined the concept in 2011 as “quantum supremacy”, which he described as “the point where quantum computers can do things that classical computers can’t, that these tasks are useful or not”. (Since then, many researchers in the field have moved on to calling it “quantum advantage,” to avoid echoes of the “white supremacy” useful task, which it hasn’t.)

Preskill’s words suggested that achieving a quantum advantage would be a turning point, marking the beginning of a new technological era in which physicists would begin to design useful tasks for quantum computers. Indeed, people had so eagerly anticipated this 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 a quantum advantage for their machines, the sense of achievement has grown murkier. For one thing, the quantum advantage does not mark the end of a race between quantum and classical computers. It’s the beginning.

Each claim of a quantum advantage has prompted other researchers to develop faster classical algorithms to challenge that claim. In the case of Google, its researchers performed 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 only took 200 seconds. A month later, IBM researchers argued that Google was using the classic bad algorithm for comparison, and that a supercomputer should only take 2.5 days. In 2021, a team using the Sunway TaihuLight supercomputer in China showed that it could complete the task in 304 seconds-just a bit slower than Google’s quantum computer. An even bigger supercomputer could run the algorithm in a few tens of seconds, says physicist Pan Zhang of the Chinese Academy of Sciences. This would put the classic computer back on top.

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