Last month, physicists at Toronto-based startup Xanadu published a strange experiment in Nature where they generated seemingly random numbers. During the pandemic, they built a tabletop machine called Borealis, consisting of lasers, mirrors and over 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 they crossed the prisms. In the end, the machine generated 216 numbers at a time – a number corresponding to the photon number in each respective beam.
Borealis is a quantum computer, and according to Xanadu researchers, this laser-powered dice roll is beyond the ability of classical or non-quantum computation. 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 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 by a specified task. The experiment “pushes the boundaries of machines we can build,” said physicist Nicolas Quesada, a member of the Xanadu team now working at Polytechnique Montréal.
“This is a major technological advance,” said Laura García-Álvarez of Chalmers University of Technology in Sweden, who was not involved in the experiment. “This device has performed a calculation that is thought to be tough on classic computers. But that does not mean useful commercial quantum computing.”
So what exactly does Xanadu’s claim of quantum advantage mean? Caltech physicist John Preskill invented the concept in 2011 as “quantum domination,” which he has described as “the point at which quantum computers can do things that classical computers cannot, whether or not these tasks are useful.” (Since then, many researchers in the field have gone on to call it “quantum advantage” to avoid echoes of “white supremacy.” Xanadu’s paper actually calls it “quantum computational advantage” because they believe “quantum advantage” implies that the computer useful task – which it did not.)
Preskill’s words suggested that gaining quantum advantages would be a turning point, marking the beginning of a new technological era in which physicists would begin to devise useful tasks for quantum computers. In fact, people predicted the milestone so hotly that the first claim about a quantum computer surpassing a classic computer – by Google researchers in 2019 – was leaked.
But as more researchers claim quantum benefits for their machines, the significance of performance has become more unclear. First, quantum advantages do not mark the end of a race between quantum computers and classical computers. That’s the beginning.
Each claim of quantum advantage has prompted other researchers to develop faster classical algorithms to challenge this claim. In the case of Google, its researchers performed a random number-generating experiment similar to Xanadus. They wrote that it would take a state-of-the-art supercomputer 10,000 years to generate a collection of numbers while it only took their quantum computer 200 seconds. A month later, researchers at IBM claimed that Google used the wrong classic 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 them could complete the task in 304 seconds– just a hair slower than Google’s quantum computer. An even larger supercomputer could execute the algorithm in dozens of seconds, says physicist Pan Zhang from the Chinese Academy of Sciences. That would put the classic computer on top again.