Quantum computers can perform certain calculations at remarkable speeds, yet connecting them over long distances has been one of the major obstacles to building large, reliable quantum networks.
Until recently, two quantum computers could only link through a fiber cable over a span of a few kilometers. This limitation meant that a system on the University of Chicago's South Side campus could not communicate with one in the Willis Tower, even though both are located within the same city. The distance was simply too great for current technology.
A new study published on November 6 in Nature Communications by University of Chicago Pritzker School of Molecular Engineering (UChicago PME) Asst. Prof. Tian Zhong suggests that this boundary can be pushed dramatically farther. His team's work indicates that quantum connections could, in theory, extend up to 2,000 km (1,243 miles).
With this method, the UChicago quantum computer that once struggled to reach the Willis Tower could instead connect with a device located outside Salt Lake City, Utah.
"For the first time, the technology for building a global-scale quantum internet is within reach," said Zhong, who recently received the prestigious Sturge Prize for this research.
Why Quantum Coherence Matters
To create high-performance quantum networks, researchers must entangle atoms and maintain that entanglement as signals travel through fiber cables. The greater the coherence time of those entangled atoms, the farther apart the connected quantum computers can be.
In the new study, Zhong's team succeeded in raising the coherence time of individual erbium atoms from 0.1 milliseconds to more than 10 milliseconds. In one experiment, they achieved 24 milliseconds of coherence. Under ideal conditions, this improvement could enable communication between quantum computers separated by roughly 4,000 km, the distance between UChicago PME and Ocaña, Colombia.
Building the Same Materials in a New Way
The team did not switch to unfamiliar or exotic materials. Instead, they reimagined how the materials were constructed. They produced the rare-earth doped crystals required for quantum entanglement using a method called molecular-beam epitaxy (MBE) rather than the standard Czochralski method.
"The traditional way of making this material is by essentially a melting pot," Zhong said, referring to the Czochralski approach. "You throw in the right ratio of ingredients and then melt everything. It goes above 2,000 degrees Celsius and is slowly cooled down to form a material crystal."
Afterward, researchers carve the cooled crystal chemically to shape it into a usable component. Zhong likens this to a sculptor chiseling away at marble until the final form emerges.
MBE relies on a very different idea. It resembles 3D printing, but at the atomic scale. The process lays down the crystal in extremely thin layers, eventually forming the exact structure needed for the device.
"We start with nothing and then assemble this device atom by atom," Zhong said. "The quality or purity of this material is so high that the quantum coherence properties of these atoms become superb."
Although MBE has been used in other areas of materials science, it had not previously been applied to this type of rare-earth doped material. For this project, Zhong collaborated with materials synthesis specialist UChicago PME Asst. Prof. Shuolong Yang to adapt MBE to their needs.
Institute of Photonic Sciences Prof. Dr. Hugues de Riedmatten, who was not part of the study, described the results as an important step forward. "The approach demonstrated in this paper is highly innovative," he said. "It shows that a bottom-up, well-controlled nanofabrication approach can lead to the realization of single rare-earth ion qubits with excellent optical and spin coherence properties, leading to a long-lived spin photon interface with emission at telecom wavelength, all in a fiber-compatible device architecture. This is a significant advance that offers an interesting scalable avenue for the production of many networkable qubits in a controlled fashion."
Preparing for Real-World Tests
The next phase of the project is to determine whether the improved coherence times can indeed support long-distance quantum communication outside of theoretical models.
"Before we actually deploy fiber from, let's say, Chicago to New York, we're going to test it just within my lab," Zhong said.
The team plans to link two qubits housed in separate dilution refrigerators ("fridges") inside Zhong's laboratory using 1,000 kilometers of coiled fiber. This step will help them verify that the system behaves as expected before moving to larger scales.
"We're now building the third fridge in my lab. When it's all together, that will form a local network, and we will first do experiments locally in my lab to simulate what a future long-distance network will look like," Zhong said. "This is all part of the grand goal of creating a true quantum internet, and we're achieving one more milestone towards that."
