PDH Feedforward Method for Improving Coherence and Gate Fidelity in Optical Qubits

Laser phase noise is one of the limiting factors that dictate the gate fidelities and coherence time in trapped-ion quantum systems. Previously, it has been reported that when the Rabi frequency of the ion qubit is close to the servo bump phase-noise frequency, the driving laser limits the fidelity and coherence time. This issue has typically been mitigated by setting the Rabi frequency away from the bump frequency, at the expense of limited servo bandwidth and ultimate fidelity. To address this issue, we developed an active phase-noise stabilization for the barium ion optical qubit laser at 1762 nm, employing a fiber electro-optic modulator (EOM) with an electrical feedforward servo. Our results demonstrate that this setup based on PDH feedforward method can suppress 15 dB servo bump phase noise near the bump peak frequency in our locking system. The laser phase noise is analyzed using delayed self-heterodyne interferometry (DSHI). We further tested the stabilized laser on the optical qubit, observing a clear improvement in coherence time based on the measured amplitude decay of Rabi oscillations even when the Rabi frequency lies within the servo bump bandwidth. This technique can be readily adapted to other optical qubits with minimal modifications.