IBM Tests Quantum Links Between Two Large Processor Chips
Four superconducting connections carried quantum information between selected qubits. Their transfer fidelity and remote-gate fidelity describe different parts of a modular computer.
Published October 29, 2026Intelligence / Communication / Quantum Communication
A quantum processor cannot be enlarged simply by connecting another chip with an ordinary data cable. The link must preserve quantum information while allowing operations that involve both modules. IBM’s October 27 account of a preprint submitted on October 21 reports progress on that connection: four long-distance couplers, or L-couplers, between two 120-qubit Nighthawk processors. A qubit is a physical carrier of quantum information. Only a selected subset around the connections was wired and measured in this experiment. 1 2
Each link includes an approximately 80-centimeter superconducting aluminum cable and chip elements that control coupling to it. The connections were assembled after mounting the modules in the cryostat, the apparatus maintaining their very low operating temperature. This tests a relevant connection arrangement; it does not demonstrate a complete 240-qubit operating computer. 3
Mean state-transfer fidelity was 99.00 percent. State transfer moves quantum information between the dedicated link qubits. Fidelity measures agreement with the intended quantum operation under the checking procedure. It is not a success rate for every computation that might use the connection. 3
A remote controlled-Z, or CZ, gate performs a joint quantum operation on qubits in separate modules. Its mean fidelity was 94.75 percent. That operation involves more than cable transmission. The percentages describe different operations; useful communication must also support sufficiently accurate operations built on it. 3
The appeal of modularity is practical: a designer can divide a large machine into processors and then connect them. That division creates interfaces that must be manufactured, installed and operated consistently. A good isolated link is therefore an intermediate result. An assembled system also needs to show what happens when many links and local operations are scheduled together, including whether one operation changes the conditions experienced by another.
Error correction adds a further requirement. It uses additional physical resources and repeated operations to protect encoded quantum information. The cost depends on the errors produced by the underlying hardware and the way they combine. These four links do not demonstrate fault tolerance, the ability to sustain computation despite component errors. Nor does their existence establish a useful application advantage. They address a hardware connection needed on the route to a larger machine. 1
The next consequential evidence will come from integrating such connections into a more fully operated system: repeatable installation, simultaneous use, stable performance and the resources required for protected operations. The result narrows a specific engineering uncertainty about joining large chips. Its significance rests on that concrete progress, without adding together the chips’ nominal qubit counts and treating the sum as a demonstrated computer.