gate using a bichromatic quadrupole field*

Two-qubit entangling gates are essential for quantum computing. We demonstrate a promising approach combining the advantages of two well-known laser-based trapped-ion gate mechanisms, the light-shift gate and the Mølmer–Sørensen gate, which produce a σ_zσ_z and a σ_φσ_φ interaction, respectively. Each scheme has unique advantages. The former acts on the computational basis and can therefore be combined with spin-echo sequences to yield high resilience against a large class of gate errors, while the latter can be trivially implemented on clock-qubits and the laser used to drive gates can be employed for single-qubit rotations as well.

We demonstrate a laser-based two-qubit entangling gate, on the optical quadrupole transition of ⁸⁸Sr⁺, inspired by the idea first proposed in [1], and subsequently demonstrated in the context of laser-free gates [2]. Using a bichromatic laser field, which can conventionally only drive a σ_φσ_φ interaction, we realise a σ_zσ_z gate with the same beam configuration but by a particular choice of detuning and Rabi frequency. Further we discuss the features, including the clock-qubit compatibility, and limitations of this new gate scheme.

[1]: Sutherland et al., New Journal of Physics, 21(3):33033, 2019

[2]: Srinivas et al., Nature, 597(7875), 209–213, 2021