IBM’s plan to build larger quantum computers depends on the complex engineering task of making multiple chips work together. IBM researchers have demonstrated a way to connect two superconducting quantum processors through four long-distance links, achieving 99% average fidelity when transferring quantum states between them, according to results the company detailed Tuesday.

The research used two 120-qubit IBM Quantum Nighthawk R1 chips connected by four L-couplers, specialized interconnects that allow quantum information to travel between processors. Each L-coupler includes an 80-centimeter superconducting aluminum cable and associated hardware. The experiment took place inside a dilution refrigerator, a cooling system that keeps quantum hardware at temperatures close to absolute zero.

A key advance is how the cables attach to the chips. IBM says earlier interconnect designs weren’t well suited to linking large processors at scale. In the new design, cables attach perpendicular to the chip surface, leaving room for more connections, and can be installed after the processors are mounted, before the system is cooled. This arrangement could make it easier to add additional processors for larger quantum systems.

The researchers also tested whether the links could support quantum operations between the two Nighthawk processors. They performed controlled-Z (CZ) gates, operations involving pairs of qubits, with an average fidelity of 94.75% across the four links. Using these gates, they created entangled states spanning the processors, including two independent entangled pairs at the same time.

These cross-chip operations are important for building modular quantum computers, which must coordinate calculations across processors and eventually support quantum error correction. However, the experiment involved only a subset of the 240 qubits on the two chips, and IBM acknowledges that further improvements are needed before the technology can support larger systems.

IBM is developing the interconnects alongside other parts of its modular quantum architecture. In August, the company demonstrated another step by connecting two cryogenic cooling modules into a shared low-temperature environment. IBM now plans to reduce errors and test additional links between larger processors. It aims to connect processors in a system containing at least 1,000 programmable qubits by 2027, ahead of its planned fault-tolerant IBM Quantum Starling system in 2029.