Weaving Quantum Processors out of Laser Light

The entanglement structure of a large-scale quantum processor made of light. (Source: S. Yokoyama)

An inter­national team of scientists from Australia, Japan and the United States has produced a prototype of a large-scale quantum processor made of laser light. Based on a design ten years in the making, the processor has built-in scala­bility that allows the number of quantum components – made out of light – to scale to extreme numbers.

Quantum computers promise fast solutions to hard problems, but to do this they require a large number of quantum compo­nents and must be relatively error free. Current quantum processors are still small and prone to errors. This new design provides an alter­native solution, using light, to reach the scale required to eventually outperform classical computers on important problems.

“While today’s quantum processors are impressive, it isn’t clear if the current designs can be scaled up to extremely large sizes,” notes Nicolas Meni­cucci, Chief Inves­tigator at the Centre for Quantum Computation and Communi­cation Technology (CQC2T) at RMIT University in Melbourne, Australia. “Our approach starts with extreme scala­bility – built in from the very beginning – because the processor, called a cluster state, is made out of light.”

A cluster state is a large collection of entangled quantum components that 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. In the two decades since they were proposed, all previous demons­trations of cluster states have failed on one or both of these counts,” says Menicucci. “Ours is the first ever to succeed at both.”

To make the cluster state, specially designed crystals convert ordinary laser light into squeezed light, which is then weaved into a cluster state by a network of mirrors, beam­splitters and optical fibres. The team’s design allows for a relatively small experiment to generate an immense two-dimen­sional cluster state with scala­bility built in. Although the levels of squeezing – a measure of quality – are currently too low for solving practical problems, the design is compatible with approaches to achieve state-of-the-art squeezing levels.

The team says their achieve­ment opens up 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”, says Hidehiro Yonezawa, Chief Inves­tigator, CQC2T at UNSW Canberra. “Our experiment demons­trates that this design is feasible and scalable.” (Source: RMIT U.)

Reference: W. Asavanant et al.: Generation of time-domain-multiplexed two-dimensional cluster state, Science 366, 373 (2019); DOI: 10.1126/science.aay2645

Link: Centre for Quantum Computation and Communication Technology CQC2T, RMIT University, Melbourne, Australia

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