Record-breaking work from Feng's group to pave the way for cryogenic computing

Very cold computing is a very hot topic—but its development has been slowed by the enormous difficulty of working with temperatures just a hair above absolute zero (0° Kelvin, or −273° Celsius). Now, Milton Feng’s group has published a pair of papers that will push cryogenic computing forward by presenting a record-breaking optical data interconnect, which the team has already demonstrated in the context of a complete system.

What’s the point of computing at such extreme temperatures? The classical computing architecture is reaching its performance limits, and cryogenic computing—in which extreme cold enables either superconductivity or maintenance of quantum state, depending on the kind of system—offers an opportunity to move beyond them.

Unfortunately, cryogenic computing can’t work without an effective way to transmit data between the cold processor and much warmer system components—while limiting heat’s ability to pass through the data connection and disastrously warm up the processor.

“That’s one of the key challenges,” explains Wenning Fu, who is an ECE Ph.D. student of Feng’s and recent winner of ECE's 2021–2022 Nick and Katherine Holonyak, Jr. Graduate Student Award for excellence in work on semiconductor optoelectronics and high-speed microelectronics. “The goal is to have a high-speed, energy-efficient, and also low-thermal-conductivity data interconnect between a 4°K processor and a room-temperature memory and other circuits. Optical interconnects are perfect candidates.”

The first paper, which is the cover story on its issue of Applied Physics Letters, was lead-authored by Haonan Wu. Wu is another ECE Ph.D. student of Feng’s and the recent winner of ECE’s 2021–2022 P. D. Coleman Outstanding Research Award for excellence in work in electromagnetics or physical and quantum electronics. To provide the high-speed data interconnect needed for cryogenic computing, the paper presents the world’s first demonstrated semiconductor laser, as an integrated optical transmitter, that functions below 4°K—the temperature at which helium becomes a liquid. Indeed, the researchers showed that it could work in temperatures as low as 2.6°K.

“The creativity of our work is that we not only make a semiconductor laser lase at liquid helium temperature, but also we developed wide-temperature-range optical and electrical packaging for the device, so that there’s no external setup required in the cryogenic environment to guide the optical signal outside of the cryogenic environment,” Wu explains. “So it’s a packaged semiconductor laser that can operate at temperatures from 2.6°K to room temperature.” 

The second paper, which likewise is the cover story on its issue of the IEEE Journal of Quantum Electronics, was lead-authored by Fu. It takes some big steps beyond the contributions of the first paper. For one thing, it reports even higher data transfer rates: a semiconductor laser performance over 50 gigabits per second (Gb/s) near 4°K. But on top of that, it also incorporates a superconducting processor based on single quantum flux (SQF) technology together with the laser at 4°K, and describes a successful demonstration, in Feng’s HMNTL lab, of a full optical fiber data linkage system with a data transfer rate of up to 20 Gb/s.

“It actually goes from the 4°K superconducting processor all the way to the room-temperature electronics,” says Fu.

The work has been supported chiefly by the Intelligence Advanced Research Projects Activity (IARPA) and the Army Research Office (ARO), but other agencies have made important contributions. Notably, the researchers used a cryostat provided by the National Institute of Standards and Technology (NIST), a superconducting processor from HYPRES, and stimulated modules from the U.S. Navy’s Naval Information Warfare Center (NIWC) Pacific to demonstrate the 20 Gb/s data transfer rate from the processor to room-temperature electronics.

Feng, who is the Nick Holonyak, Jr., Endowed Chair Emeritus in Electrical and Computer Engineering, says, “We are very proud of this work as the first demonstration of a full high-speed NRZ [non-return-to-zero] error-free data transmission optical link from 4^oK to room temperature. And we expect that cryogenic oxide-VCSEL [vertical-cavity surface-emitting laser] optical fiber links will be able to deliver greater than 200 gigabits per second NRZ for the most energy-efficient data transfer rate toward approximately one femtojoule per bit.”

The two papers are:

• H. Wu, W. Fu, M. Feng, and D. Deppe, “2.6K VCSEL Data Link for Cryogenic Computing,” Applied Physics Letters, 119, 041101, 2021. https://doi.org/10.1063/5.0054128
• W. Fu, H. Wu, and M. Feng, “Superconducting Processor Modulated VCSELs for 4K High-Speed Optical Data Link,” IEEE Journal of Quantum Electronics, 58(2), April 2022. https://doi.org/10.1109/JQE.2022.3149512