Jul 14, 2022

Experiments between quantum and classical computers show term’s evolving meaning. Enlarge / Xanadu's quantum chip. 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 tabletop machine named Borealis, consisting of lasers, mirrors, and over a kilometer of optical fiber. Within Borealis, 216 beams of infrared light bounced around through a complicated network of prisms. Then, a series of detectors counted the number of photons in each beam after they traversed the prisms. Ultimately, the machine generated 216 numbers at a time—one 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 is beyond the capability of classical, or non-quantum, computing. It took Borealis 36 microseconds to generate one set of 216 numbers from a complicated statistical distribution. They estimated 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 so-called quantum advantage, where a quantum computer defeats a state-of-the-art supercomputer at a specified task. The experiment "pushes the boundaries of machines we can build," says physicist Nicolas Quesada, a member of the Xanadu team who now works at Polytechnique Montréal. "This is a great technological advance," says Laura García-Álvarez of Chalmers University of Technology in Sweden, who was not involved in the experiment. "This device has performed a computation that is believed hard for classical computers. But it does not 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 where quantum computers can do things that classical computers can't, regardless of whether those tasks are useful." (Since then, many researchers in the field switched to calling it "quantum advantage," to avoid echoes of "white supremacy." Xanadu's paper actually calls it "quantum computational advantage" because they think "quantum advantage" implies that the computer performed a useful task—which it didn't.) Advertisement 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 devising useful tasks for quantum computers. Indeed, people anticipated the milestone so hotly that the first claim of a quantum computer outperforming a classical computer— by Google researchers in 2019 —was leaked. But as more researchers claim quantum advantage for their machines, the meaning of the achievement has become murkier. For one thing, quantum advantage doesn't mark the end of a race between quantum and classical computers. It's the beginning. Each claim of quantum advantage has set off other researchers to develop faster classical algorithms to challenge that claim. In Google's case, its researchers performed a random-number-generating experiment similar to Xanadu's. They wrote that it would take a state-of-the-art supercomputer 10,000 years to generate a collection of numbers, while it took their quantum computer only 200 seconds. A month later, researchers at IBM 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 —just a hair slower than Google's quantum computer. That same year, the developers of that algorithm presented an even faster method. A larger 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. That would put the classical computer on top again. "If you say you've gotten quantum advantage, you're saying that no one will ever 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 that claim. And big claims require strong evidence." A 2020 quantum advantage claim from researchers at the University of Science and Technology in China met similar criticism. The team, led by physicist Pan Jian-Wei, also used their quantum computer to generate numbers according to a set probability distribution. In their paper, they claimed that their 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 a supercomputer 73 days.