Scientists at the Southern University of Science and Technology (SUSTech), the International Quantum Academy, and other institutions in China have developed low-loss interconnects for connecting the various modules of modular superconducting quantum processors. These interconnects are built on impendence transformers and pure aluminium wires.
To construct his innovative low-loss interconnects, Zhong custom-ordered pure aluminium coaxial cables and merged them with on-chip impedance transformers. The resultant interconnects were easier to wire-bond to quantum processors and had a much lower loss (by one order of magnitude) than conventional interconnects built using NbTi cables.
Zhong attempted to connect two quantum computers in one of his earlier research using NbTi superconducting wires, frequently used to design cryogenic/quantum systems. He tried to wire-bond the quantum chips directly to the connecting NbTi cable to lessen the connection loss (i.e., energy loss naturally occurring when energy went from one processor to the other through the cables).
"I found that this was quite difficult, so I came up with the idea of trying new cables made of different superconducting metals, such as aluminum, the same material as our quantum circuits. Coaxial cables made with pure aluminum are not readily available on the shelf, because aluminum is more lossy and difficult to solder than copper, making it unsuitable for normal cabling applications. Moreover, its superconducting transition temperature is below the liquid Helium temperature. Other than quantum interconnection applications, it's rare to find scenarios where a pure aluminum coaxial cable is needed," said Youpeng Zhong, one of the researchers who carried out the study.
The most recent study by this group of scientists demonstrates the enormous potential of aluminium cables for creating efficient interconnects to connect processing modules in modular quantum computers. Soon, other modular systems may use the low-loss connection that Zhong and his coworkers developed, advancing continuing attempts to design more scalable quantum computers.
"Pure aluminum cables turned out to be the perfect choice for quantum interconnects. Our interconnects include the custom developed aluminum coaxial cable, wire-bond connection between the cable and the quantum chip and a quarter-wavelength transmission line on the quantum chip, which serves as an impedance transformer. The impedance transformer in the team's interconnect converts the wire-bond connection point to a current node of a standing wave mode that is used to transfer quantum states. This significantly minimizes the resistive loss at the point of connection between different quantum processors,” explained Zhong.
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