Laser-free trapped-ion entangling gates with an oscillating magnetic-field gradient at radio frequency*

We demonstrate a recently proposed method for trapped-ion entangling gates implemented using an oscillating magnetic-field gradient at radio frequency in addition to two microwave magnetic fields symmetrically detuned about the qubit frequency [1]. This technique enables laser-free entangling gates with reduced sensitivity to qubit frequency errors. The experiment is performed in a surface-electrode trap that incorporates current-carrying electrodes to generate the microwave fields and the oscillating magnetic field gradient. Currently, we achieve a Bell-state fidelity of 0.996(2) with ground-state-cooled ions and 0.991(3) for ions cooled to the Doppler limit (nbar=2). The radio-frequency currents used to generate the gradient also give rise to a tunable differential ac Zeeman shift on the two ions which can be turned on and off. While the gate is insensitive to this shift, it can be used independently to perform single ion addressing. This method of addressing does not require additional control fields or rotation of the ion crystal. We can combine this addressing with an entangling gate to create any Bell state from a given initial state.

[1] Sutherland et al., New J. Phys.21, 033033 (2019)