Last month, physicists from Toronto-based startup Xanadu published a curious experiment in Nature in which they generated seemingly random numbers. During the pandemic, he built a tabletop machine called the Borealis, which contained lasers, mirrors and more than a kilometer of optical fiber. Within Borealis, 216 beams of infrared light bounced around through an intricate network of prisms. Then, a series of detectors counted the number of photons in each beam after it passed the prism. Ultimately, the machine generated 216 numbers at once—a number corresponding to the photon count in each respective beam.
Borealis is a quantum computer, and according to the Xanadu researchers, this laser-powered dice roll goes beyond the capability of classical, or non-quantum, computing. It took Borealis 36 microseconds to generate a set of 216 numbers from a complex statistical distribution. He estimated that it would take an average of 9,000 years for Fugaku, the most powerful supercomputer at the time of the experiment, to produce a set of numbers from a uniform distribution.
The experiment is the latest in a series of demonstrations of so-called quantum advantage, where a quantum computer beats a state-of-the-art supercomputer at a specific 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.
“This is a great technological advancement,” says Laura García-Alvarez of Chalmers University of Technology in Sweden, who was not involved in the experiment. “This device has performed a calculation that is known to be difficult for classical computers. But it is by no means 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 described as “the point where quantum computers can do things classical computers cannot, even if those tasks are useful.” be.” (Since then, many researchers in the field have called it “quantum advantage” to avoid echoes of “white supremacy.” Useful work—which it didn’t.)
Preskill’s words suggested that achieving quantum advantage would be a turning point, marking the beginning of a new technological era, in which physicists would begin to devise useful functions for quantum computers. In fact, people anticipated the milestone so hotly that the first claim that a quantum computer outperformed a classical computer – by Google researchers – was leaked in 2019.
But as more researchers claim quantum advantages for their machines, the meaning of the achievement has become unclear. For one thing, quantum advantage does not mark the end of a race between quantum and classical computers. This is the beginning.
Each claim of quantum advantage has prompted other researchers to increasingly develop classical algorithms to challenge that claim. In Google’s case, its researchers conducted a random-number-generating experiment similar to Xanadu. He wrote that a state-of-the-art supercomputer would take 10,000 years to create a collection of numbers, while his quantum computer took only 200 seconds. A month later, IBM researchers argued that Google used the wrong classical algorithm for comparison, and that a supercomputer should take just 2.5 days. In 2021, a team using the Sunway TaihuLight supercomputer in China showed they could complete the task in 304 seconds—only a hair slower than Google’s quantum computer. In the same year, the developers of that algorithm presented an even faster method. A large supercomputer would be able to execute this algorithm in dozens of seconds, says physicist Pan Zhang of the Chinese Academy of Sciences, who helped develop both algorithms. This would put the classical computer on top again.
“If you say that you have achieved quantum advantage, you are saying that no one will simulate your experiment as accurately as your experiment was,” says physicist Jacob Bulmer of the University of Bristol. “It’s a big scientific moment when you make this claim. And big claims require strong evidence.”
The 2020 quantum advantage claim by researchers at the University of Science and Technology in China has faced similar criticism. The team, led by physicist Pan Jian-Wei, also used their quantum computer to generate numbers according to a defined probability distribution. In his paper, he claimed that his quantum computer could generate a set of numbers in 200 seconds, while the world’s most powerful supercomputer would take 2.5 billion years. In January, Bulmer led a team to show that it would actually take 73 days for a supercomputer.