A prototype of a large-scale quantum processor made of laser light has been created by a team from Australia, Japan, and the U.S. The processor has built-in scalability that allows the number of quantum components to scale to extreme numbers. “Our approach starts with extreme scalability, built in from the very beginning, because the processor, called a cluster state, is made out of light,” said Nicolas Menicucci, chief investigator at the Centre for Quantum Computation and Communication Technology (CQC2T) at RMIT University. The entanglement structure of a large-scale quantum processor made of light. Courtesy of Shota Yokoyama 2019. A cluster state is a large collection of entangled quantum components — a key resource for measurement-based quantum computing. The cluster state performs quantum computations when measured in a particular way. “To be useful for real-world problems, a cluster state must be both large enough and have the right entanglement structure,” Menicucci said. Universal quantum computing would require cluster states that are both large and possess (at least) a two-dimensional topology. To create a cluster state, the researchers used specially designed crystals to convert ordinary laser light into squeezed light. They weaved the squeezed light into a cluster state using a network of mirrors, beamsplitters, and optical fibers. The structure of the cluster state is two-dimensional and consists of a square lattice that was tailored to a highly scalable, time-multiplexed experimental platform. It is compatible with Bosonic error-correcting codes that, with higher squeezing levels, could enable fault-tolerant quantum computation, the researchers said. A network of optical devices — mirrors, beamsplitters, and optical fibers — weave laser light into an optical quantum processor. Courtesy of CQC2T. The processor’s design enables it to generate an immense two-dimensional cluster state with scalability built in, even in a relatively small experiment. Although the levels of squeezing are currently too low for solving practical problems, the team said its design is compatible with techniques used to achieve state-of-the-art squeezing levels. The research could lead to new possibilities for quantum computing with light. “In this work, for the first time in any system, we have made a large-scale cluster state whose structure enables universal quantum computation,” said Hidehiro Yonezawa, chief investigator, CQC2T at University of New South Wales (UNSW) Canberra. “Our experiment demonstrates that this design is feasible and scalable.” The research was published in Science (https://doi.org/10.1126/science.aay2645).