Compact all-optical atomic magnetometer for operation in unshielded environments

Biomagnetic sensing is often carried out in magnetically shielded settings, whereas broader portable and clinical applications require magnetometers that remain sensitive and stable under geomagnetic-field conditions. At the same time, high-density biomagnetic array measurements, including multichannel magnetoencephalography, benefit from miniaturized sensor heads that enable close sensor placement and scalable channel counts. Here, we demonstrate a miniaturized, single-beam, all-optical cesium atomic magnetometer based on an anti-relaxation-coated vapor cell. A coaxial architecture aligns the pump and probe beams along a shared optical axis within the sensor head and is implemented using an integrated 3D-printed optical assembly. The sensor-head dimensions are reduced from 270 × 240 × 50 to 85.3 × 65.2 × 30.9 mm³, corresponding to an ∼19-fold reduction in volume, while maintaining intrinsic sensitivity. Under a bias field of ∼ 50000 n T, corresponding to geomagnetic-field-level operation, the magnetometer achieves an intrinsic sensitivity of 36 fT/Hz at 10√Hz. Measurements performed in shielded, outdoor, and unshielded indoor environments further demonstrate robust operation under different magnetic-noise conditions. These results demonstrate the feasibility of compact, array-compatible atomic-magnetometer sensor heads for biomagnetic measurements beyond strictly shielded conditions.