A recent study published in Nature showcases an innovative breakthrough in quantum computing that connects two quantum processing units (QPUs) using real-time classical communication.
Study: Combining quantum processors with real-time classical communication. Image Credit: UladzimirZuyeu/Shutterstock.com
The researchers demonstrated the use of error-mitigated dynamic circuits across 142 qubits, enabling quantum gates to be controlled based on mid-circuit measurements. This approach enhanced qubit connectivity, allowing multiple processors to work together seamlessly and significantly advancing the field of modular quantum computing.
Tackling Scalability and Connectivity Challenges
Scalability is a critical challenge in quantum computing, as expanding quantum processors often introduces hardware constraints and connectivity issues. To address this, researchers explored modular architectures that utilize virtual gates for long-range qubit interactions.
Among the tested methods, Local Operations and Classical Communication (LOCC) proved superior to simpler Local Operations (LO) by leveraging teleportation circuits with virtual Bell pairs. LOCC reduced the complexity of quantum circuit compilation while maintaining high performance, making it the preferred approach for modular systems.
In this study, the team implemented graph states with periodic boundary conditions on IBM’s Eagle processors to test various connectivity strategies. Three methods were compared: SWAP gates, LO, and LOCC.
While SWAP gates introduced significant errors due to the increased use of controlled-NOT (CNOT) gates, LO and LOCC demonstrated much lower error rates. Notably, LOCC achieved a 99 % confidence level in detecting bipartite entanglement across all graph edges, surpassing SWAP in stabilizer quality and error metrics.
Dynamic Circuit Integration with Modular Processors
To push the limits of modular quantum computing, the team combined two 127-qubit Eagle processors into a 254-qubit modular system using real-time classical links. This integration enabled dynamic quantum circuit execution, where mid-circuit measurements informed subsequent operations. The researchers successfully created a 134-qubit graph state with periodic boundaries, implementing long-range gates with both LO and LOCC.
The experiments demonstrated that LOCC maintained entanglement across all graph edges, outperforming benchmarks that lacked long-range gates. While LOCC required additional resources, such as cut Bell pairs generated through parameterized quantum circuits, it provided robust entanglement and error mitigation. These results highlight the effectiveness of dynamic circuits in overcoming connectivity constraints and advancing modular architectures.
Innovative Circuit Cutting Techniques
A key contribution of this study was the use of circuit cutting, which partitions large quantum circuits into smaller subcircuits for individual execution. The researchers introduced a "cut Bell pair factory" using parameterized quantum circuits to generate Bell pairs for teleportation. These Bell pairs enabled long-range gates such as CNOT to be performed on spatially separated qubits.
To reduce the overhead typically associated with circuit cutting, the team optimized the quantum process decomposition (QPD) framework, which expresses quantum channels as weighted sums of simpler channels. The experiments leveraged dynamic circuits with advanced error mitigation techniques, including dynamical decoupling and zero-noise extrapolation, to address latency and noise issues during real-time operations.
Pre-experimental benchmarking ensured the selection of high-quality qubits with minimal relaxation and measurement errors, enhancing the reliability of the graph state construction. Stabilizer measurements on smaller qubit chains provided valuable insights into hardware performance, guiding the implementation of scalable quantum experiments.
Implications and Future Directions
This research establishes a strong foundation for scalable and error-mitigated quantum computing using modular architectures. The integration of real-time classical communication and dynamic circuits represents a significant step forward, enabling multiple processors to function as a unified system.
The successful creation of a 134-qubit graph state demonstrates the feasibility of large-scale quantum experiments, opening new pathways for applications such as Hamiltonian simulation and measurement-based quantum computing.
By addressing connectivity and resource challenges with innovative techniques like LOCC, cut Bell pairs, and dynamic circuit execution, this study showcases a roadmap for advancing modular quantum computing. Future work could explore further optimizations in circuit cutting and error mitigation to expand the capabilities of quantum processors and support increasingly complex algorithms.
Journal Reference
Carrera Vazquez, A., et al. (2024). Combining quantum processors with real-time classical communication. Nature, 1-5. DOI: 10.1038/s41586-024-08178-2, https://www.nature.com/articles/s41586-024-08178-2
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