Researchers at the University of Virginia developed a scalable quantum computing platform, which drastically reduces the number of devices needed to achieve quantum speed, on a photonic chip the size of a penny.

Xu Yi, assistant professor of electrical and computer engineering at the University of Virginia (UVA) School of Engineering and Applied Science, focused on generating quantum modes, which span the full spectrum of variables between one and zero. Yi turned to fields of light, which are also a full spectrum; each lightwave in the spectrum has the potential to become a quantum unit. Yi hypothesized that by entangling fields of light, the light would achieve a quantum state. To realize this, Yi used multiplexing, a technique often used in optical communications to enable multiple signals or colors to be used in parallel.

In 2014, Olivier Pfister, professor of quantum optics and quantum information at UVA, led research that generated more than 3000 quantum modes in a bulk optical system. Using this many quantum modes, however, requires a very large footprint to contain the thousands of mirrors, lenses, and other components necessary to run an algorithm and perform operations.

“The future of the field is integrated quantum optics,” Pfister said. “Only by transferring quantum optics experiments from protected optics labs to field compatible photonic chips will bona fide quantum technology be able to see the light of day. We are extremely fortunate to have been able to attract to UVA a world expert in quantum photonics such as Xu Yi, and I’m very excited by the perspectives these new results open to us.”

Yi’s group created a quantum source in an optical microresonator, a ring-shaped millimeter-size structure that envelops the photons and generates a microcomb, which efficiently converts photons from single to multiple wavelengths. Light circulates around the ring to build up optical power. This power buildup enhances the chance for photons to interact, which produces quantum entanglement between fields of light in the microcomb.

Through multiplexing, Yi’s researchers verified the generation of 40 qumodes from a single microresonator on a chip, proving that multiplexing of quantum modes can work in integrated photonic platforms. This is just the number they were able to measure.

“We estimate that when we optimize the system, we can generate thousands of qumodes from a single device,” Yi said.

The multiplexing method paves a path toward quantum computing for real-world conditions where errors are inevitable. The number of qubits needed to compensate for errors could exceed one million, with a proportional increase in the number of devices. Multiplexing reduces the number of required devices by two or three orders of magnitude.

The team’s system offers two additional advantages in the quantum computing quest. Quantum computing platforms that use superconducting electronic circuits require cryogenic cooling. Because photons have no mass, quantum computers with photonic integrated chips can run or sleep at room temperature. Additionally, the microresonator fabricated by Hansuek Lee, assistant professor at the Korean Advanced Institute of Science and Technology, used standard lithography techniques, which means that the resonator or quantum source can be mass-produced.

“We are proud to push the frontiers of engineering in quantum computing and accelerate the transition from bulk optics into integrated photonics,” Yi said. “We will continue to explore ways to integrate devices and circuits in a photonics-based quantum computing platform and optimize its performance.”

The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-25054-z).