Optical Quantum Processor Scales to Extreme

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).