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Modularity meets quantum engineering
Engineers at the Grainger College of Engineering, University of Illinois Urbana-Champaign, have demonstrated a practical modular architecture for superconducting quantum processors that can be linked and reconfigured with very high fidelity. Engineers have built modular quantum components that connect with near-perfect fidelity, unlocking scalable and reconfigurable quantum systems. Credit: Shutterstock
The concept is simple to describe and hard to achieve in practice: instead of building one monolithic quantum processor containing thousands or millions of qubits, assemble smaller, high-quality modules that can be linked together on demand. This modular approach mirrors how LEGO bricks combine to form complex structures, but for quantum computers the challenge is maintaining quantum coherence and precise control across physical boundaries.
Why modularity matters for quantum scaling
Traditional, monolithic superconducting quantum processors face limits in fabrication yield, thermal management, and wiring complexity. Small defects or variability in a large chip can degrade overall performance. Modularity addresses these issues by allowing researchers to combine independently optimized units, upgrade hardware, and replace failing modules without scrapping an entire processor. Crucially, modular designs must preserve quantum gate fidelity and enable error detection and correction to move toward fault-tolerant operation.
In their paper published in Nature Electronics, the Illinois team reports a modular superconducting system that connects separate qubit devices using superconducting coaxial cables. The links allow qubits on different modules to interact and perform two-qubit gates with fidelity close to that of on-chip operations. The reported SWAP-gate fidelity is approximately 99%, corresponding to under 1% error for the operation — a threshold that makes modular scaling more practical.
Technical approach and key results
The researchers built two independent superconducting devices and connected them with low-loss coaxial superconducting cables acting as quantum interconnects. By engineering couplers and timing carefully, they achieved coherent exchange of quantum states between modules and implemented high-fidelity SWAP operations. Fidelity here quantifies how closely the implemented quantum operation matches the ideal operation; a fidelity of 1.0 would indicate a perfect, error-free gate.
"We've created an engineering-friendly way of achieving modularity with superconducting qubits," said Wolfgang Pfaff, assistant professor of physics and senior author of the paper. He emphasized the need not only to make high-quality entangling operations across modules but also to be able to disassemble and reconfigure systems for testing and repair.
The experiment demonstrates that cable-based links can reach numbers that justify scaling: the cable connection preserved coherence and delivered entanglement-quality interactions across physical modules. This opens routes for building larger processors by stitching modules together rather than relying solely on ever-larger monolithic chips.
Implications for fault tolerance and quantum networks
High-fidelity modular links are a step toward fault-tolerant quantum computing. Fault tolerance requires multiple layers: qubits with long coherence times, precise single- and two-qubit gates, reliable qubit connectivity, and robust error detection and correction. Modular architectures can simplify some aspects of error correction by localizing errors to individual modules and enabling hot-swappable replacements.
Additionally, cable-based modularity informs designs for quantum networks and distributed quantum computing where separate processors exchange quantum information across links. The approach complements other quantum interconnect strategies such as photonic links or microwave-to-optical transducers and may be particularly advantageous for superconducting platforms.
Expert Insight
Dr. Maria Hernandez, a quantum systems engineer at a national laboratory, commented: "Reaching ~99% SWAP fidelity across modules is a meaningful milestone. It shows that the practical engineering constraints of interconnects—loss, impedance matching, and thermal anchoring—can be addressed while preserving quantum coherence. The next challenge is integrating error detection and scaling the number of linked modules without introducing cross-talk or control overhead that negates the benefits of modularity."
Next steps and challenges
The Illinois team plans to extend the experiment to connect more than two modules while maintaining the ability to detect and correct errors. Scaling will require careful system-level engineering: multiplexed control lines, cryogenic packaging, low-loss interconnects, and software protocols for distributed gate scheduling and error tracking. Researchers will also compare cable-based approaches with other interconnect technologies to determine optimal trade-offs for large-scale quantum systems.
Conclusion
Modular superconducting quantum processors that connect via superconducting coaxial cables represent a promising path toward scalable, reconfigurable, and upgradeable quantum computers. By demonstrating near-99% SWAP gate fidelity between separate devices, the Grainger College of Engineering team has provided a practical blueprint for stitching together larger quantum systems while retaining gate quality—an important advance on the road to fault-tolerant quantum computing.
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