Breakthrough could connect quantum computers at 200X the distance

Quantum computers are powerful, lightning-fast, and notoriously difficult to connect to each other over long distances.

Previously, the maximum distance between two quantum computers via fiber optic cable was a few kilometers. This means that even if a fiber optic cable were installed between them, the quantum computers on the University of Chicago’s South Side campus and Willis Tower in downtown Chicago would be too far apart to communicate with each other.

Research published today in Natural communications from the Pritzker School of Molecular Engineering at the University of Chicago (UChicago PME) Asst. Professor Tian Zhong would theoretically extend this maximum to 2,000 km (1,243 miles).

With Zhong’s approach, that same UChicago quantum computer that previously couldn’t reach Willis Tower could now connect and communicate with a quantum computer outside Salt Lake City, Utah.

“For the first time, the technology to build a global quantum internet is within our reach,” said Zhong, who recently received the prestigious Sturge Prize for this work.

Connecting quantum computers to create powerful, high-speed quantum networks involves entangling atoms via fiber optic cable. The longer these entangled atoms maintain their quantum coherence, the longer the distance between these quantum computers.

In the new paper, Zhong and his team at UChicago PME increased the quantum coherence times of individual erbium atoms from 0.1 milliseconds to more than 10 milliseconds. In one case, they demonstrated up to 24 milliseconds, which would theoretically allow quantum computers to connect over a staggering 4,000 km, the distance between UChicago PME and Ocaña, Colombia.

Same materials, different method

The innovation was not in using new or different materials, but in constructing the same materials in a different way. They created the rare earth-doped crystals needed to create quantum entanglement using a technique called molecular beam epitaxy (MBE) rather than the traditional Czochralski method.

“The traditional way of making this material is basically using a crucible,” Zhong said of the Czochralski method. “You add the right ratio of ingredients and then melt it all. The temperature rises above 2,000 degrees Celsius and is slowly cooled to form a crystal.”

To transform the crystal into a computer component, researchers then “sculpt” it chemically to give it the necessary shape. It’s similar to how a sculptor might select a slab of marble and eliminate everything that isn’t the statue.

MBE, however, is more akin to 3D printing. It sprays thin layer after thin layer, building the necessary crystal into its exact final shape.

“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 is a known technique, it has never been used to construct this form of rare earth doped material. Zhong and his team worked with materials synthesis expert UChicago PME Asst. Professor Shuolong Yang will adapt MBE for this purpose.

“The approach demonstrated in this paper is very innovative,” said Professor Hugues de Riedmatten of the Institute of Photonic Sciences, a world leader in the field who was not involved in the research.

“This shows that a well-controlled, bottom-up nanofabrication approach can lead to the realization of unique rare earth ion qubits with excellent optical properties and spin coherence, leading to a long-lived spin photon interface with emission at the telecommunications wavelength, all in a fiber-compatible device architecture. This is a significant advance that provides an attractive scalable path for the production of numerous qubits that can be networked in a controlled manner. “

Next steps

Zhong and his team will next test whether the increased coherence time allows quantum computers to connect to each other over long distances.

“Before we deploy optical fiber from, say, Chicago to New York, we will test it only in my laboratory,” Zhong said.

This involves connecting two qubits in separate dilution refrigerators (“fridges”), both located in Zhong’s lab at UChicago PME, via 1,000 kilometers of coiled cable. This is the next step, but far from the last.

“We are currently building the third refrigerator in my laboratory. When everything is together, it will form a local network, and we will first do experiments locally in my laboratory to simulate what a future long-distance network will look like,” Zhong said. “This is all part of the big goal of creating a true quantum Internet, and we are taking another step in that direction.”