IBM Links Two 120-Qubit Quantum Processors With Four Meter-Scale Superconducting Cables
The links moved quantum states between chips with 99% average accuracy and survived three warm-and-cool cycles, a step toward building quantum computers out of modules.
IBM researchers have joined two large superconducting quantum processors into one working system using four separate high-quality cables, the first time such a multi-link connection has been demonstrated between chips of this size. The team described the work in a preprint on arXiv, led by Neereja M. Sundaresan, under the title "Meter-scale quantum interconnects between large quantum processors."
The problem they set out to solve is how to make quantum computers bigger. Many researchers believe the best route is modular: build several smaller processors and link them so they act as one machine. The links have to carry fragile quantum information without destroying it. For superconducting chips, that means building reliable connections around a meter long inside refrigerators chilled to just above absolute zero.
Until now, links between superconducting chips have mostly been a single connection between small experimental processors. Those allowed researchers to transfer quantum states from one chip to another and to create entanglement. Bigger machines will need many connections installed after the processors are already mounted in their refrigerators. The attachments must be packed tightly, lose very little signal, and keep working as the system is repeatedly warmed and cooled.
IBM linked two of its 120-qubit Nighthawk R1 processors. In each link, a qubit on one chip passes its information to a dedicated interface qubit. An adjustable coupler then feeds that information into an 80-centimeter, or 31-inch, superconducting aluminum cable, which carries it to a matching set of components on the other chip. To pack the cables more densely, the team plugged each one into its chip from directly above rather than from the side.
Across the four links, quantum states were transferred with an average accuracy of 99 percent, and the best link reached 99.29 percent. Two-qubit logic operations spanning both processors averaged 94.75 percent accuracy. Using those operations, the researchers created entangled states shared across both chips, including 4-qubit and 6-qubit states, and generated two entangled pairs at the same time through neighboring links. All four connections stayed operational through three cycles of warming and cooling.
The thermal-cycling result matters as much as the accuracy figures. A quantum computer that works once but fails after being warmed to room temperature for maintenance and cooled again would be impractical to service. Surviving three such cycles with all four links intact suggests the top-down plug-in design can tolerate the stresses of repeated temperature swings, though three cycles is a small number compared with what a deployed machine would face.
The result is not yet commercially practical. Phys.org reported that further improvements will be needed, especially in how long the quantum information lasts. Even so, it marks a significant step toward computers in which quantum information is stored and processed across many processors, which would make future machines far easier to expand and maintain.
