Free induction decay gradiometer based on 87Rb MEMS vapor cell in the geomagnetic field range

We describe a magnetically silent gradiometer based on the 87Rb micro-electro-mechanical systems vapor cell with an internal volume of 0.1 cm3. The gradiometer measures the magnetic field by probing the free induction-decay frequency in the time domain, thereby eliminating feedback loop interference and RF crosstalk. Experimental results demonstrated that a differential sensitivity of 300 fT/Hz1/2 can be realized with a millimeter-scale vapor cell, even in the geomagnetic field range, which is consistent with the theoretical differential Cramér–Rao lower bound. To characterize magnetometer noise sources, in-phase and quadrature resonance curves were recorded using a lock-in amplifier, identifying single-channel photon shot noise of approximately 180 fT/Hz1/2 as the primary limitation of the device sensitivity. The compact and array-compatible design facilitates portable quantum sensing applications in unshielded environments.