Four superconducting cables link two large quantum processors into one

Introduction

Modular quantum computing promises to scale beyond the limits of a single chip by interconnecting multiple processors. A recent experimental breakthrough demonstrates that four meter‑scale superconducting cables can reliably link two large 120‑qubit processors, preserving quantum information with high fidelity. This achievement marks a concrete step toward practical, expandable quantum machines.

Building the quantum interconnect

The IBM team tackled three engineering hurdles:

1. Length and cryogenic environment – Each aluminum cable is 80 cm long and operates just above absolute zero, requiring careful thermal anchoring to avoid heat leaks.
2. Dense packaging – Rather than attaching cables from the side, the researchers inserted each cable from directly above the chip, allowing tighter spacing without compromising access to other components.
3. Robustness through thermal cycles – All four links survived three full warm‑up and cool‑down cycles, demonstrating that the connections can endure the repeated handling typical of large‑scale systems.

In each link, a qubit on the first processor transfers its state to a dedicated interface qubit, which couples to the cable via an adjustable coupler. The signal travels the cable and is received by a matching interface on the second processor, where it is handed off to a target qubit.

Performance metrics

The experiment measured two key figures of merit:

• State‑transfer fidelity – Across the four links the average fidelity was 99 %, with the best link reaching 99.29 %.
• Inter‑processor two‑qubit gate fidelity – Logical operations that span both chips achieved an average fidelity of 94.75 %.

Using these high‑quality links, the team generated entangled states that spanned both processors, including four‑ and six‑qubit entangled states and simultaneous generation of two entangled pairs via neighboring links.

Implications for scalable quantum computers

While the results are promising, further work is needed before modular architectures become commercially viable. Key challenges include extending the coherence time of quantum information across the links and integrating many more connections without degrading performance. Nonetheless, the demonstrated ability to install multiple high‑fidelity links after the chips are already mounted suggests a practical pathway for building larger quantum systems that can be expanded and serviced more easily than monolithic designs.

Conclusion

The successful deployment of four superconducting cables linking two sizable quantum processors showcases that modular quantum computing is moving from theory toward reality. By achieving near‑perfect state transfer and respectable two‑qubit gate fidelities across meter‑scale interconnects, the work lays a foundation for future machines where many processors cooperate as a single, scalable quantum computer.