Large-scale, Multi-pass, Two-chamber RF Atomic Magnetometer

We describe one of the largest radio-frequency RF atomic magnetometers presently operating. A total atomic volume of 128 cm³, with correspondingly large number of ⁸⁷Rb atoms, can reduce atom noise. A total of 44 passes of the probe beam reduces photon-shot noise. The atomic vapor is divided between two chambers allowing for pumping of the cells individually; doing so with opposite-helicity light enables use as an intrinsic gradiometer. In this configuration, common-mode noise sources including light-shift noise can be reduced. Magnetic tuning fields can also be applied to the chambers individually, allowing simultaneous measurement of two frequencies. An application of this is in the search for contraband materials using Nuclear Quadrupole Resonance (NQR), for which simultaneous measurement can significantly reduce search times. We demonstrate dual-frequency measurement on an effective range of 423-531 kHz, corresponding to the NQR frequencies of ammonium nitrate NH₄NO₃ at the lowest value and potassium chlorate KClO₃ at the highest. We explore fundamental, as well as instrumental, noise contributions to the sensitivity in this system.