Unlocking a fast adiabatic CZ gate and exact residual $ZZ$ cancellation between fixed-frequency transmons using a floating tunable coupler

TL;DR

This paper demonstrates a symmetric floating tunable coupler enabling fast, high-fidelity adiabatic CZ gates with exact residual ZZ cancellation in superconducting transmon qubits, using pulse shaping to minimize errors.

Contribution

It introduces a symmetric floating tunable coupler architecture that naturally achieves fast, high-fidelity CZ gates with exact residual ZZ cancellation, overcoming previous trade-offs.

Findings

Achieved a 24 ns adiabatic CZ gate with over 99.9% fidelity.

Demonstrated stable operation over several hours.

Implemented pulse-shaping techniques to suppress non-adiabatic errors.

Abstract

Tunable couplers in superconducting qubit architectures enable strong qubit-qubit interactions for two-qubit gates while suppressing unwanted coupling during single-qubit operations. However, achieving low error rates for fast two-qubit gates remains challenging, as suppressing leakage and non-adiabatic errors typically requires specialized qubit, coupler, or pulse designs, often at the expense of an idling $ZZ=0$ condition. In this work, we demonstrate that a symmetric floating tunable coupler provides a natural platform for fast, high-fidelity adiabatic controlled-Z (CZ) gates. Its favorable energy-level structure eliminates the conventional trade-off between rapid conditional-phase accumulation and adiabatic evolution while preserving exact cancellation of residual $ZZ$ interaction at idling. This architecture exhibits intrinsic robustness to non-adiabatic transitions, even under simple flux modulation waveforms. To push performance at short gate durations, where maintaining adiabaticity becomes more challenging despite the favorable level structure, we introduce pulse-shaping techniques based on the instantaneous adiabatic factor that further suppress non-adiabatic errors. We experimentally realize a 24 ns adiabatic CZ gate with fidelity exceeding 99.9% and stable operation over several hours.