Pulse sequences for fast trapped-ion gates relying on relative phase stability*

Trapped ions make promising qubits thanks to their long coherence times and uniformity across ions of the same species. However, high-fidelity multi-qubit gates using trapped ions are one of the slowest basic operations required for quantum information processing, potentially leading to high latency in complex algorithms. Previous theoretical1 and experimental2 work has shown that gates as fast as 1.6 us based on the σz-σz interaction can be executed with high fidelity if one of the Raman lasers used to drive the gate is appropriately amplitude-modulated to satisfy a set of well-established constraints. Among these constraints is insensitivity to the initial relative phase of the Raman laser beams used to drive the gate, which is difficult to control using free-space laser control.

Here we consider fast, amplitude-modulated σz-σz gates without this initial Raman beam phase requirement. We demonstrate a computational framework that models these gates and optimizes relevant experimental parameters to find suitable pulse sequences that achieve fast operation with high fidelity. These calculations can inform future fast gate implementations, e.g. using integrated photonic addressing, where the relative phase of the Raman beams may potentially be controlled with interferometric precision, relaxing the constraint on the initial Raman beam phase, and ideally reducing gate infidelity and/or laser power requirements.

(1) A M Steane et al., New J. Phys. 16 053049 (2014)

(2) V Schafer et al., “Fast Gates and Mixed-Species Entanglement with Trapped Ions.” PhD thesis, University of Oxford (2018)