Angle-robust two-qubit gates in a linear ion crystal

In trapped-ion quantum computers, two-qubit entangling gates are generated by applying spin-dependent force which uses phonons to mediate interaction between the internal states of the ions. To maintain high-fidelity two-qubit gates under fluctuating experimental parameters, robust pulse-design methods are applied to remove the residual spin-motion entanglement in the presence of motional mode-frequency drifts. Here we propose an improved pulse-design method that also guarantees the robustness of the two-qubit rotation angle against uniform mode-frequency drifts by concatenating pulses with opposite sensitivity of the angle to mode-frequency drifts. We experimentally verify significantly improved robustness of the rotation angle against uniform modefrequency drifts, as well as observe an improvement in gate fidelity from 97.84(10)% to 98.11(11)%, compared to a single frequency-modulated pulse.

Automated summary

In trapped-ion quantum computers, spin-dependent force and phonons are used to generate two-qubit entangling gates. To maintain high fidelity under fluctuating experimental parameters, robust pulse-design methods are used. An improved method is proposed that guarantees the robustness of the rotation angle against uniform mode-frequency drifts. Experimental results show significantly improved robustness and gate fidelity compared to a single frequency-modulated pulse.