Dual-Axis Closed Loop of a Single-Beam Atomic Magnetometer: Toward High Bandwidth and High Sensitivity

Atomic magnetometers operated in spin-exchange relaxation-free (SERF) have the potential to replace superconducting quantum interference devices (SQUIDs) in the field of biomagnetism. Here, we demonstrate the dual-axis closed-loop control of a single-beam SERF magnetometer that allows sensing of two orthogonal magnetic fields simultaneously and independently with suppressed crosstalk effect, achieved by applying a rotating modulation field and using the negative feedback. Operating in the dual-axis closed-loop mode with a miniature 4 $\times $ 4 $\times $ 4 mm 87Rb cell, we achieve the magnetic field sensitivity of 20 fT/Hz1/2 and a bandwidth of 1.8 kHz. In addition to the experimental study of the sensitivity and bandwidth, we perform a theoretical analysis of the dual-axis response and develop an effective decoupling method. The implementation of closed-loop control enables the magnetometer to be robust against the disturbance of the ambient field, which could enlarge the dynamic range, enhance the systematic stability, and suppress the crosstalk effect. Such a simple, yet high-performance SERF magnetometer could be suitable as a promising candidate in the field of biomagnetism, allowing for a denser sensor array and opening up new possibilities for noninvasive biomagnetic imaging.