Spin-Feedback-Enhanced Atomic Magnetic Gradiometer

Feedback-based spin control has recently attracted increasing attention, with initial implementations in systems such as spin masers and comagnetometers. Here, we demonstrate the first implementation of spin feedback in dual-cell spin-exchange relaxation-free (SERF) gradiometers, achieving a significant enhancement in overall performance. We analyze the stability boundary for spin feedback theoretically, which enables the amplification of spin response while ensuring self-oscillation-free operation. The differential magnetic signal experimentally benefits from the enhanced response and shows a significant improvement in the time domain. Moreover, introducing the feedback factor as an additional parameter enables delicate control of the phase response between two channels, facilitating broadband phase matching across gradiometer channels with different intrinsic parameters and magnetic-field environments.