Modular Quantum Processor with an All-to-All Reconfigurable Router

Superconducting qubits provide a promising approach to large-scale fault-tolerant quantum computing. However, qubit connectivity on a planar surface is typically restricted to only a few neighboring qubits. Achieving longer-range and more flexible connectivity, which is particularly appealing in light of recent developments in error-correcting codes, however, usually involves complex multilayer packaging and external cabling, which is resource intensive and can impose fidelity limitations. Here, we propose and realize a high-speed on-chip quantum processor that supports reconfigurable all-to-all coupling with a large on-off ratio. We implement the design in a four-node quantum processor, built with a modular design comprising a wiring substrate coupled to two separate qubit-bearing substrates, each including two single-qubit nodes. We use this device to demonstrate reconfigurable controlled-<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>Z</a:mi></a:math> gates across all qubit pairs, with a benchmarked average fidelity of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mn>96.00</c:mn><c:mo>%</c:mo><c:mo>±</c:mo><c:mn>0.08</c:mn><c:mo>%</c:mo></c:math> and best fidelity of <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:mn>97.14</e:mn><e:mo>%</e:mo><e:mo>±</e:mo><e:mn>0.07</e:mn><e:mo>%</e:mo></e:math>, limited mainly by dephasing in the qubits. We also generate multiqubit entanglement, distributed across the separate modules, demonstrating GHZ-3 and GHZ-4 states with fidelities of <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"><g:mn>88.15</g:mn><g:mo>%</g:mo><g:mo>±</g:mo><g:mn>0.24</g:mn><g:mo>%</g:mo></g:math> and <i:math xmlns:i="http://www.w3.org/1998/Math/MathML" display="inline"><i:mn>75.18</i:mn><i:mo>%</i:mo><i:mo>±</i:mo><i:mn>0.11</i:mn><i:mo>%</i:mo></i:math>, respectively. This approach promises efficient scaling to larger-scale quantum circuits and offers a pathway for implementing quantum algorithms and error-correction schemes that benefit from enhanced qubit connectivity. Published by the American Physical Society 2024