Creating a Quantum Computer Network

Duke Engineering launched the Beyond the Horizon initiative to provide interdisciplinary teams with substantial investment to begin pursuing extremely high-risk, high-reward projects that have the potential for deep, transformative societal impact. Three proposals were selected for an initial round of funding that will play key roles in shaping Duke Engineering’s future research and teaching profile. Each play to the school’s unique strengths and hold the promise of helping to define the future of their respective fields.

It’s been a while since Tingjun Chen studied quantum mechanics; 12 years ago as an undergrad, to be exact. An assistant professor of electrical and computer engineering (ECE) at Duke, Chen usually works in the classical physics realm in topics related to next-generation wireless networking.

But through conversations with colleagues at the Duke Quantum Center (DQC), Chen realized his expertise in fiber optics might unlock the ability to create a quantum computer network.  

Chen and his collaborators, including Jungsang Kim, the Schiciano Family Distinguished Professor of ECE; Norbert Linke, professor of physics; and John Board, associate professor of ECE, have teamed up to increase quantum computing power using fiber optics. Now, after more than a decade since college, Chen is brushing up on quantum physics as he begins a promising partnership.

“Since I arrived at Duke in 2020, Jungsang and Norbert have taught me a lot about what their technologies can do and how they can be plugged into large-scale networks, which is the focus of my research,” Chen said.

Overall Goals

The most powerful classical computers today connect hundreds or thousands of smaller computers together in one network. Similarly, more powerful quantum computers in the future will be made up of many smaller quantum computers linked together, Kim said.

“At some point, you can’t build a bigger computer,” Kim said. “They don’t build silicon chips the size of a football field. Instead, you construct a building the size of a football field and fill it with hundreds of thousands of connected computers.”

A quantum network would allow its researchers to run increasingly complicated computations. One potential application for a quantum network is running fully end-to-end encrypted protocols that cannot be broken by classical computers.

“Multiple quantum computers could run fundamentally secure computations, where the quantum data center could execute a job, and they don’t even know what they did for you,” Kim said.

Talking between two quantum computers isn’t as simple as plugging in an ethernet cable, though. The fastest method to transmit data on classical computers is through fiber optics, which are long, thin strands of glass that communicate data encoded as light signals. However, the process of converting data from a memory unit to a communication unit is more complicated in quantum computers than classical ones, and they require separate fiber optic channels.

Fortunately for this team, the trapped-ion system used at DQC lends itself to transmitting its data through fiber optics. At its large, and growing, hub in downtown Durham, DQC researchers use finely tuned lasers to excite ions during experiments. Those lasers, in turn, carry quantum information encoded in light particles known as photons. In theory, they will be able to transmit those photons along fiber optics to be deciphered by another quantum computer at the other end.

“This is not just an engineering challenge, as we must also delve into the fundamentals of quantum mechanics,” Linke said. “We are creating an entangled quantum state spanning the city of Durham, with particles downtown and on campus connected by what Einstein referred to as ‘spooky action at a distance.’”

However, there are obstacles still to overcome. Light signals are known to degrade as they travel further distance in fiber optics, an issue Chen’s lab has studied extensively. They will also have to test its efficacy with environmental uncertainties from the real world.

“Imagine if there are construction sites digging near the fiber optic cables, or the temperature changes, or we get a lot of rain – that will affect the polarization of the fiber optics, and we won’t know the effect of those factors until we see them,” Chen said.

The Duke Difference

Few academic institutions have the capability to build a quantum network at this scale given the high level of hardware and expertise required.

Duke’s Office of Information Technology (OIT) maintains an extensive fiber optic network that connects DQC with the East and West campuses, a network spanning hundreds of miles in Durham. More importantly, they have been generous and supportive in sharing this resource with faculty to facilitate research projects.

“We have a long tradition at OIT of making our network an asset to researchers,” said Board, who also serves as associate chief information officer at OIT. “We are constantly attempting to future proof our infrastructure against new demands, so our job is not just to keep the trains running, but to build better trains.”

This project represents the collaborative spirit common at Duke that pushes the boundaries of science and technology.

“The expertise of our faculty, the collegiality and most importantly, the funding support allows us the time and opportunity to think boldly about the future of quantum networks and that is very exciting,” Chen said.