Atomic Magnetometer Multisensor Array for rf Interference Mitigation and Unshielded Detection of Nuclear Quadrupole Resonance

An array of four Rb-87 vector magnetometers is used to detect nuclear quadrupole resonance signals in an unshielded environment at 1 MHz. With a baseline of 25 cm, the length of the array, radio-frequency interference mitigation is also demonstrated; a radio-station signal is suppressed by a factor of 20 without degradation to the signal of interest. With these compact sensors, in which the probe beam passes through twice, the fundamental limit to detection sensitivity is found to be photon-shot noise. More passes of the probe beam overcome this limitation. With a sensor of similar effective volume, 0.25 cm(3), but 25 x more passes, the sensitivity is improved by an order of magnitude to 1.7 +/- 0.2 fT/root Hz