Noise reduction in optically pumped magnetometer assemblies

In most magnetic field measurement configurations the resolution of optically pumped magnetometers is limited by the shot noise of the pump light. However, in practice this noise limit is overwhelmed by other sources. One of them is the conversion of the pump laser’s frequency modulation (FM) noise to amplitude modulation (AM) noise due to the absorption in the magnetometer’s alkali vapour cell. This extra noise can be nearly completely cancelled by the illumination of an additional cell with the same laser light and the subtraction of its photo current from that of the measurement cell. The correlation of the photo signals of different cells is just slightly decreased by the applied measurement and rf fields B0 and B1, respectively. As a result, in real magnetic field measurements using the photo-current subtraction, a noise-limited magnetic field resolution of just twice the shot-noise limit can be achieved. This is experimentally shown for the most thrifty setup with two cells; one time using the second cell just for the photo-current subtraction, the other time also serving for magnetic field measurements, forming a gradiometer with the first cell. Yet, the photo-current subtraction method is most appealing for magnetometer arrays, where the photo signal of just one additional vapour cell can be used for the noise reduction of the complete array.