Large-scale multipass two-chamber rf atomic magnetometer

Optically pumped atomic magnetometers are capable of measuring radio-frequency magnetic fields with high sensitivity, but their narrow bandwidth, of the order of 1 kHz, restricts their applications. Further, their full potential is often not realized, with noisy environments or quantum noise curtailing their sensitivity. This study presents a two-chamber atomic magnetometer, with independent control over the tuning field and pumping helicity in each chamber, which addresses both bandwidth and sensitivity limitations. It demonstrates a practical instance of bandwidth expansion, by simultaneous measurement of signals spaced more than 100 kHz apart, at frequencies corresponding to nuclear quadrupole resonance of ammonium nitrate and potassium chlorate, two materials commonly found in improvised explosive devices. It further demonstrates: the intrinsic reduction of common-mode noise, including environmental and probe light-shift noise, down to 2.4 $\mathrm{fT}/\sqrt{\mathrm{Hz}}$, with the use of resonant, but oppositely pumped, atomic chambers; the reduction of photon shot noise down to 28 $\mathrm{aT}/\sqrt{\mathrm{Hz}}$ with the use of 44 probe passes through the two chambers; and the reduction of atom noise down to 34 $\mathrm{aT}/\sqrt{\mathrm{Hz}}$, in agreement with prediction, using a large combined volume, 128 ${\mathrm{cm}}^{3}$, of ${}^{87}\mathrm{Rb}$ atoms. This work underlines the potential sensitivity of these quantum sensors, while exploring the technical limitation, namely the high pump power required, that stands as a barrier to that sensitivity. Further, it marks a significant step towards utilizing atomic magnetometers in applications demanding broader bandwidth or multiplexing, notably simultaneous measurements at far-spaced radio frequencies.