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Bulletin of the American Physical Society

Towards a 4-local coupler for superconducting flux qubits - part 2: distinguishing multi-spin interactions from lower-order effects

Thomas R. Bergamaschi, Tim Menke, Cyrus F. Hirjibehedin, Steven J. Weber, Andrew J. Kerman, William D. Oliver

Published Mar 18, 2021• 0 citations
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The field of quantum information has matured and various protocols implementing a quantum computer are being pursued. Most similar to a classical computer is the circuit model. In 2007 Aharonov et al. showed the equivalence between the circuit model and a quantum annealer, and with this proofed the universality of quantum annealing. Here the system starts in an easily preparable ground state and evolves adiabatically to a final ground state which yields the solution of the computational problem. However, equivalence with the circuit model requires multi-local interactions, i.e. interaction terms involving more than two subsystems. Natural interactions are only two-local, hence the construction or simulation of higher order couplers is indispensable for a universal quantum annealer. Also, four-local couplers allow compact implementation of error correction with the Bacon-Shor code. Four-local interactions can further serve as a tool for basic research. Here we show that in a specific flux qubit coupler design without ancilla qubits, four body interactions are induced by virtual coupler excitations. For specific parameter regimes they are even the leading effect and can be tuned up to the GHz range.

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Superconducting qubits have achieved exceptional gate fidelities, exceeding the error-correction threshold in recent years. One key ingredient of such improvement is the introduction of tunable couplers to control the qubit-to-qubit coupling through frequency tuning. Moving toward fault-tolerant quantum computation, increasing the number of physical qubits is another step toward effective error correction codes. Under a multiqubit architecture, flux control (Z) lines are crucial in tuning the frequency of the qubits and couplers. However, dense flux lines result in magnetic flux crosstalk, wherein magnetic flux applied to one element inadvertently affects neighboring qubits or couplers. This crosstalk obscures the idle frequency of the qubit when flux bias is applied, which degrades gate performance and calibration accuracy. In this study, we characterize flux crosstalk and suppress it in a multiqubit-coupler chip with multi-Z lines without adding additional readout for couplers. By quantifying the mutual flux-induced frequency shifts of qubits and couplers, we construct a cancellation matrix that enables precise compensation of non-local flux, demonstrating a substantial reduction in Z-line crosstalk from 56.5‰ to 0.13‰, which is close to statistical error. Flux compensation corrects the CZ SWAP measurement, leading to a symmetric map with respect to flux bias. Compared with a crosstalk-free calculated CZ SWAP map, the measured map indicates that our approach provides a near-zero crosstalk for the coupler-transmon system. These results highlight the effectiveness of our approach in enhancing flux crosstalk-free control and supporting its potential for scaling superconducting quantum processors.

APL Quantum•Feb 18, 2026•2 citations
The field of quantum information has matured and various protocols implementing a quantum computer are being pursued. Most similar to a classical computer is the circuit model. In 2007 Aharonov et al. showed the equivalence between the circuit model and a quantum annealer, and with this proofed the universality of quantum annealing. Here the system starts in an easily preparable ground state and evolves adiabatically to a final ground state which yields the solution of the computational problem. However, equivalence with the circuit model requires multi-local interactions, i.e. interaction terms involving more than two subsystems. Natural interactions are only two-local, hence the construction or simulation of higher order couplers is indispensable for a universal quantum annealer. Also, four-local couplers allow compact implementation of error correction with the Bacon-Shor code. Four-local interactions can further serve as a tool for basic research. Here we show that in a specific flux qubit coupler design without ancilla qubits, four body interactions are induced by virtual coupler excitations. For specific parameter regimes they are even the leading effect and can be tuned up to the GHz range.
Superconducting qubits have achieved exceptional gate fidelities, exceeding the error-correction threshold in recent years. One key ingredient of such improvement is the introduction of tunable couplers to control the qubit-to-qubit coupling through frequency tuning. Moving toward fault-tolerant quantum computation, increasing the number of physical qubits is another step toward effective error correction codes. Under a multiqubit architecture, flux control (Z) lines are crucial in tuning the frequency of the qubits and couplers. However, dense flux lines result in magnetic flux crosstalk, wherein magnetic flux applied to one element inadvertently affects neighboring qubits or couplers. This crosstalk obscures the idle frequency of the qubit when flux bias is applied, which degrades gate performance and calibration accuracy. In this study, we characterize flux crosstalk and suppress it in a multiqubit-coupler chip with multi-Z lines without adding additional readout for couplers. By quantifying the mutual flux-induced frequency shifts of qubits and couplers, we construct a cancellation matrix that enables precise compensation of non-local flux, demonstrating a substantial reduction in Z-line crosstalk from 56.5‰ to 0.13‰, which is close to statistical error. Flux compensation corrects the CZ SWAP measurement, leading to a symmetric map with respect to flux bias. Compared with a crosstalk-free calculated CZ SWAP map, the measured map indicates that our approach provides a near-zero crosstalk for the coupler-transmon system. These results highlight the effectiveness of our approach in enhancing flux crosstalk-free control and supporting its potential for scaling superconducting quantum processors.