Building rare-earth-doped crystals using a different manufacturing technique could increase the range of quantum networks from a few kilometres to a potential 2,000 kilometres, bringing a quantum internet closer than ever, according to research that shatters previous distance records for quantum computer connections.
Researchers at the University of Chicago Pritzker School of Molecular Engineering raised the quantum coherence times of individual erbium atoms from 0.1 milliseconds to longer than 10 milliseconds. In one instance, they demonstrated up to 24 milliseconds, which would theoretically allow quantum computers to connect at a staggering 4,000 kilometres. The findings were published in Nature Communications.
Previously, the maximum distance two quantum computers could connect through a fibre cable was a few kilometres. With the new approach, quantum computers could theoretically connect across distances equivalent to Chicago to Salt Lake City, Utah.
“For the first time, the technology for building a global-scale quantum internet is within reach,” said Assistant Professor Tian Zhong, who recently received the prestigious Sturge Prize for this work.
Entangling atoms
Linking quantum computers to create powerful, high-speed quantum networks involves entangling atoms through a fibre cable. The longer the time those entangled atoms maintain quantum coherence, the longer the distance those quantum computers can link to each other.
The innovation was not in using new or different materials, but from building the same materials a different way. The team created the rare-earth-doped crystals necessary to create quantum entanglement using a technique called molecular-beam epitaxy (MBE) rather than the traditional Czochralski method.
The traditional Czochralski method involves melting ingredients above 2,000 degrees Celsius and slowly cooling them to form a material crystal. Researchers then chemically “carve” it into the needed form, similar to how a sculptor might select a slab of marble and chip away everything that isn’t the statue.
MBE, however, is more akin to 3D-printing. It sprays thin layer after thin layer, building the needed crystal into its exact final form.
“We start with nothing and then assemble this device atom by atom,” said Professor Zhong. “The quality or purity of this material is so high that the quantum coherence properties of these atoms become superb.”
Whilst MBE is a known technique, it has never been used to build this form of rare-earth-doped material. Professor Zhong and his team worked with materials synthesis expert Assistant Professor Shuolong Yang to adapt MBE for this purpose.
The team will next test whether the increased coherence time enables quantum computers to connect over long distances, initially linking two qubits in separate dilution refrigerators through 1,000 kilometres of spooled cable within the laboratory.
