Intensity Stabilization in Fiber Amplifiers: Effects on Phase Noise, Linewidth, and Qubit Coherence

Abstract: Intensity noise is a factor limiting the coherence time of qubits in trapped ion

quantum systems. Previously, we observed that using a Thulium-doped fiber amplifier (TDFA)

introduces intensity fluctuations to the input seed laser, thus limiting the coherence time. To

address this issue, we developed an intensity stabilization setup for TDFA used for

quadrupole transition, employing an acousto-optic modulator (AOM) with an electrical feedback

servo. Our results demonstrate that this setup can reduce intensity noise by up to 20 dB from DC

to 10 kHz without introducing additional phase noise and broadening the linewidth to the input

signal. The phase noise and linewidth of the laser was analyzed using delayed self-heterodyne

interferometry (DSHI). We also cross-check the stabilized beam using a single ion in our ion trap

setup. Based on the Rabi oscillation results at different power levels, we observe an improvement

in coherence time.