A new type of quantum computer has shown that it can also reign.
A photonic quantum computer, which takes advantage of light particles or photons, has performed an impossible calculation for a conventional computer, researchers in China reported on Dec. 3 in Science. That milestone, known as quantum supremacy, was only reached once before, in 2019, by Google’s quantum computer (SN: 23/10/19). Google’s computer, however, is based on superconducting materials, not photons.
“This is the first independent confirmation of Google’s claim that you can actually achieve quantum supremacy,” says computer science theorist Scott Aaronson of the University of Texas at Austin. "That's exciting."
Named Jiuzhang according to an ancient Chinese mathematical text, the new quantum computer can perform a calculation in 200 seconds that would take more than half a trillion years on the world's fastest non-quantum or classical computer.
"My first impression was, 'wow,'" says quantum physicist Fabio Sciarrino of Sapienza University in Rome.
Google’s device, called Sycamore, is based on small quantum bits made of superconducting materials, which conduct energy without resistance. Instead, Jiuzhang consists of a complex array of optical devices that carry photons around. These devices include light sources, hundreds of beam splitters, dozens of mirrors and 100 photon detectors.
Using a process called boson sampling, Jiuzhang generates a distribution of numbers that is very difficult to reproduce for a classical computer. This is how it works: photons are first sent to a network of channels. There, each photon meets a series of beam splitters, each of which sends the photon down two paths simultaneously, in what is called a quantum superposition. Paths also merge together, and repeated division and fusion causes photons to interfere with each other according to quantum rules.
Finally, the number of photons in each of the network output channels is measured at the end. When repeated many times, this process produces a distribution of numbers based on how many photons were found at each output.
If it works with large numbers of photons and many channels, the quantum computer will produce a distribution of numbers that is too complex for a classical computer to calculate. In the new experiment, up to 76 photons traversed a 100-channel network. For one of the most powerful classical computers in the world, the Chinese supercomputer Sunway TaihuLight, predicting the results that the quantum computer would get for anything beyond about 40 photons was intractable.
Although Google was the first to break the barrier of quantum supremacy, the milestone "is not a unique achievement," says study co-author and quantum physicist Chao-Yang Lu of China University of Science and Technology in Hefei. "It's a continuous competition between constantly improved quantum hardware and constantly improved classical simulation." Following Google’s claim of quantum supremacy, for example, IBM proposed a type of calculation that could allow a supercomputer to perform the task that Google’s computer completed, at least theoretically.
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And achieving quantum supremacy does not necessarily mean that quantum computers are still very useful, because calculations are esoteric designed to be difficult for classical computers.
The result raises the profile of photon quantum computers, which have not always received as much attention as other technologies, says quantum physicist Christian Weedbrook, CEO of Xanadu, a Toronto-based company focused on building photon quantum computers. "Historically, photonics has been the dark horse."
One of the limitations of Jiuzhang, Weedbrook points out, is that it can only perform a single type of task, namely boson sampling. Instead, Google’s quantum computer could be programmed to run a variety of algorithms. But other types of photonic quantum computers, including Xanadu, are programmable.
Demonstrating quantum supremacy with a different type of device reveals how quickly quantum computing advances, says Sciarrino. "The fact that now the two platforms are able to achieve this regime … shows that the whole field is advancing in a very mature way."