For the second time ever, a quantum device has outdone a conventional computer. Chinese researchers describe in Science a photonic quantum computer performing in 200 seconds a calculation that would take more than half a billion years on the world’s fastest supercomputer. In 2019, Google was the first to demonstrate a quantum computational advantage, also known as quantum supremacy, using superconducting elements as qubits. By contrast, the Chinese Jiuzhang computer is based on photons.
Jiuzhang is essentially a complex arrangement of optical components, including light sources, beam splitters and mirrors. The experiment involves sending photons through this ‘maze’ and measuring their distribution at the other end. Because of quantum mechanical interactions along the way, this so-called boson sampling process generates an ‘unpredictable’ distribution of numbers, which is exceedingly difficult for classical computers to replicate.
Like Google’s computation, boson sampling is in itself not very useful for anything else than demonstrating quantum supremacy. Adding programmable elements to the optical array, however, will allow boson samplers to perform useful computations, such as chemical simulations. Dutch startup Quix, among other companies, is building such a programmable photonic quantum processor. Unlike Jiuzhang, which is built using discrete components, this processor is based on integrated photonics, ie combines all optical elements on a single chip.