Trapped-ion two-qubit gates with 99.99% fidelity without ground-state cooling: experiment

We introduce the *smooth gate* - a novel entangling gate method for trapped-ion qubits where residual motional errors are adiabatically eliminated by ramping the gate detuning. We combine the power of this technique with the robustness of electronic qubit control [1] to perform two-qubit gates with an estimated error of < 1×10−4 without the use of ground-state cooling [2]. We characterise the gate error using a new protocol inspired by subspace randomised benchmarking [3] which does not require the use of any single-qubit rotations. We further show that the error remains < 5 × 10−4 for Doppler-cooled ions with gate mode temperatures of up to nbar = 9.4(3). These results show that trapped-ion quantum computers can be operated above the Doppler limit, allowing for significantly faster device operation.

[1] C. M. Löschnauer et al., PRX Quantum 6, 040313 (2025)

[2] A. C. Hughes et al., arXiv:2510.17286v1 [quant-ph] (2025)

[3] R. T. Sutherland et al., arXiv:2510.09508 [quant-ph] (2025)