Quantum computers are considered promising for certain computational tasks, but so far they can only be linked to one another over very short distances. The reason is the limited coherence time of the qubits involved, which is quickly lost during transmission over optical fiber. A research team at the University of Chicago now reports an approach that significantly shifts this temporal limit—thereby extending the potential range of future quantum networks.
“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 work.
“The traditional way of making this material is by essentially a melting pot,” Zhong said of the Czochralski method. “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.”
At the heart of the work by Tian Zhong, Assistant Professor at the Pritzker School of Molecular Engineering, are rare-earth-doped crystals—specifically erbium. Such materials are well established in quantum communication because they operate at telecom wavelengths. What is new, however, is how they are made. Instead of growing the crystals conventionally from a melt using the Czochralski process, the team uses molecular beam epitaxy (MBE). In MBE, the building blocks are deposited layer by layer under ultra-high vacuum—a process that functionally resembles 3D printing, just at an atomic scale.
“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.”
The advantage of this bottom-up approach lies in the high material purity and the precise control of the crystal structure. In experiments, the team was able to increase the coherence time of individual erbium atoms from previously around 0.1 milliseconds to more than 10 milliseconds—up to 24 milliseconds in some cases. In theory, this could allow quantum computers to be linked over several thousand kilometers of optical fiber.
“The approach demonstrated in this paper is highly innovative,” said Institute of Photonic Sciences Prof. Dr. Hugues de Riedmatten, a world leader in the field who was not involved in the research. “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.”
What matters here is less a new material than the manufacturing method. While conventional crystals must be structured afterward, MBE-grown components are created directly in their target geometry. For Zhong, that is a crucial difference—similar to the difference between subtractive machining and additive manufacturing.
“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.
“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.”
As the next step, the team plans to test the improved coherence times in practice by connecting qubits in separate cryostats via kilometer-long spools of optical fiber. This brings a robust infrastructure for networked quantum computers at least experimentally closer.
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