High-precision pulse calibration of tunable couplers for high-fidelity two-qubit gates in superconducting quantum processors

TL;DR

This paper presents a novel pulse calibration method for tunable couplers in superconducting quantum processors that improves control precision and gate fidelity by directly measuring and correcting flux pulse distortions without extra readout.

Contribution

The introduced calibration scheme simplifies flux control and enhances two-qubit gate fidelities by directly measuring coupler response and applying predistortion, eliminating the need for additional coupler readout.

Findings

Achieved high-fidelity diabatic CZ and iSWAP gates with over 99.6% fidelity.

Demonstrated significant reduction in pulse distortion and phase errors.

Validated robustness of the calibration method across multilayer gates.

Abstract

For superconducting quantum processors, stable high-fidelity two-qubit operations depend on precise flux control of the tunable coupler. However, the pulse distortion poses a significant challenge to the control precision. Current calibration methods, which often rely on microwave crosstalk or additional readout resonators for coupler excitation and readout, tend to be cumbersome and inefficient, especially when couplers only have flux control. Here, we introduce and experimentally validate a novel pulse calibration scheme that exploits the strong coupling between qubits and couplers, eliminating the need for extra coupler readout and excitation. Our method directly measures the short-time and long-time step responses of the coupler flux pulse transient, enabling us to apply predistortion to subsequent signals using fast Fourier transformation and deconvolution. This approach not only simplifies the calibration process but also significantly improves the precision and stability of the flux control. We demonstrate the efficacy of our method through the implementation of diabatic CZ and iSWAP gates with fidelities of $99.61\pm0.04\%$ and $99.82\pm0.02\%$, respectively, as well as a series of diabatic CPhase gates with high fidelities characterized by cross-entropy benchmarking. The consistency and robustness of our technique are further validated by the reduction in pulse distortion and phase error observed across multilayer CZ gates. These results underscore the potential of our calibration and predistortion method to enhance the performance of two-qubit gates in superconducting quantum processors.