Building blocks and quantum computers: New research leans on modularity

(Press-News.org) What do children’s building blocks and quantum computing have in common? The answer is modularity. It is difficult for scientists to build quantum computers monolithically – that is, as a single large unit. Quantum computing relies on the manipulation of millions of information units called qubits, but these qubits are difficult to assemble. The solution? Finding modular ways to construct quantum computers. Like plastic children’s bricks that lock together to create larger, more intricate structures, scientists can build smaller, higher quality modules and string them together to form a comprehensive system.

Recognizing the potential of these modular systems, researchers from The Grainger College of Engineering at the University of Illinois Urbana-Champaign have presented an enhanced approach to scalable quantum computing by demonstrating a viable and high-performance modular architecture for superconducting quantum processors. Their work, published in Nature Electronics, expands on previous modular designs and paves the way toward scalable, fault-tolerant and reconfigurable quantum computing systems.

Monolithic superconducting quantum systems are limited in size and fidelity, which predicts scientists’ rate of success in performing logical operations. A fidelity of one signifies no mistakes; as such, researchers want to achieve a fidelity as close to one as possible. Compared to these limited monolithic systems, modularity enables system scalability, hardware upgrades, and tolerance to variability, making it a more attractive option for building system networks.

“We’ve created an engineering-friendly way of achieving modularity with superconducting qubits,” said Wolfgang Pfaff, an assistant professor of physics and the senior author of the paper. “Can I build a system that I can bring together, allowing me to manipulate two qubits jointly so as to create entanglement or gate operations between them? Can we do that at a very high quality? And can we also have it such that we can take it apart and put it back together? Typically, we only find out that something went wrong after putting it together. So we would really like to have the ability to reconfigure the system later.”

By constructing a system where two devices are connected with superconducting coaxial cables to link qubits across modules, Pfaff’s team demonstrated ~99% SWAP gate fidelity, representing less than 1% loss. Their ability to connect and reconfigure separate devices with a cable while retaining high quality provides novel insight to the field in designing communication protocols.

“Finding an approach that works has taken a while for our field,” Pfaff said. “Many groups have figured out that what we really want is this ability to stitch bigger and bigger things together through cables, and at the same time reach numbers that are good enough to justify scaling. The problem was just finding the right combination of tools.”

Moving forward, the Grainger engineers will turn their focus toward scalability, attempting to connect more than two devices together while retaining the ability to check for errors.

“We have good performance,” Pfaff said. “Now we need to put it to the test and say, is it really going forward? Does it really make sense?”

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