Abstract:

We describe a novel pulsed magnetic gradiometer based on the optical interference of sidebands generated using two spatially separated alkali vapor cells. The sidebands are produced with high efficiency using parametric frequency conversion of a probe beam interacting with 87Rb atoms in a coherent superposition of magnetically sensitive hyperfine ground states. First, experimental evidence of the sideband process is described for both steady-state and pulsed operation. Then, a theoretical framework is developed that accurately models sideband generation based on density matrix formalism. The gradiometer is then constructed using two spatially separated vapor cells, and a beat-note is generated. The frequency of the beat-note is determined by the magnetic field gradient between the two vapor cells. In contrast to traditional magnetic gradiometers, our approach provides a direct readout of the gradient field without the intermediate step of subtracting the outputs of two spatially separated magnetometers. Using this technique, we developed a compact magnetic gradiometer sensor head with integrated optics with a sensitivity of 25 fT/cm/ Hz with a 4.4 cm baseline, while operating in a noisy laboratory environment unshielded from Earth’s field. Also, operation of the sensor in magnetic fields both along the laser axis and perpendicular to the axis are discussed, leading to the potential of making the sensor Dead-Zone-Free.