Superconducting Cables Link Two Large Quantum Processors

In a significant advancement for quantum computing, a team at IBM Research has successfully linked two large superconducting processors using four separate high-quality quantum connections. This achievement, detailed in a new paper published on the arXiv preprint server, marks the first time such a feat has been accomplished, with the connections installed after both chips were already in place. This work directly addresses a key challenge in modular quantum computer design, which proposes interconnecting multiple processors to function as one larger, more powerful machine. The setup was built inside equipment chilled to just above absolute zero.

For superconducting processors, reliable quantum links measuring around a meter (3.3 feet) long are crucial to effectively carry fragile quantum information without destruction. Previously, quantum links between superconducting chips mostly involved a single connection between smaller experimental processors, allowing researchers to transfer quantum states and create quantum entanglement. However, to construct larger machines, engineers must install numerous connections between processors already mounted in their refrigerators. These attachments must be densely packed, minimize signal loss, and endure repeated warming and cooling cycles of the system.

The IBM team tackled this by linking two of its 120-qubit Nighthawk R1 processors. In each link, a qubit within one processor passed its information to a dedicated 'interface' qubit. An adjustable coupler then fed this information into an 80 cm (31-inch) superconducting aluminum cable, which carried it to a matching set of components on the other chip. To enable denser packing of these cables, the team innovatively plugged each one into its chip from directly above, rather than from the side, a design choice critical for scalability.

Researchers rigorously tested the setup, evaluating its ability to pass quantum states between chips and perform logic operations involving qubits on both processors. Across the four links, quantum states were transferred with an impressive average accuracy of 99%, with the best link reaching 99.29%. Two-qubit logic operations spanning both processors achieved an average accuracy of 94.75%. Utilizing these operations, the team successfully created entangled states shared across both processors, including 4- and 6-qubit states, and simultaneously generated two entangled pairs through neighboring links. Crucially, all four connections remained fully operational through three cycles of warming and cooling. While further improvements are needed for commercial practicality, particularly in the longevity of its quantum information, these achievements represent an important step, paving the way for future expandable and maintainable quantum computers.

What to watch: Further developments in extending quantum information longevity for practical applications.

Editor's note: The report is comprehensive and accurately reflects the provided source material, including technical specifics and performance metrics.