Bell-Bloom magnetometer based on square optical path.

We present a single-beam Bell-Bloom magnetometer using a compact, square optical path in a vapor cell with intensity modulation of the light beam. This design achieves dead-zone-free three-axis magnetic field measurements and, additionally, can determine the magnitude and direction of the magnetic field gradient. Experimental results demonstrate the dead-zone-free operation, with sensitivity in each axis scaling with the effective optical path length along that direction. Strong magnetic field gradients induce distinct resonance peak splitting, whereby the splitting width is proportional to the gradient magnitude. By applying a reference gradient, the system can infer the direction of an external gradient, effectively functioning as a dual-axis gradiometer. Rotating the vapor cell allows optimization of the optical path to minimize gradient-induced measurement distortions by aligning it perpendicular to the gradient direction.