Volume Efficient Quantum Limited Rb Radio-Frequency Magnetometer

We demonstrate a volume efficient quantum limited radio-frequency (rf) ⁸⁷Rb magnetometer, where the Zeeman resonance of alkali atoms is tuned to the rf frequency of the magnetic field being measured. We also develop an analytic model for the fundamental quantum limit on rf magnetometer sensitivity, which scales with the inverse of the square root of measurement volume. By implementing a series of experimental optimizations to better match with model conditions, our magnetometer reaches a magnetic field sensitivity of 0.36 fT/Hz^1/2 in a 1.4 ×1.4× 2.0 cm³ cell near 100 kHz, corresponding to 0.71 fT/Hz^1/2 per 1/cm^3/2. While better absolute sensitivity had been demonstrated previously elsewhere using much larger cell volume, our result gets the closest to the quantum limit determined by the geometric mean of spin-exchange and spin-destruction cross-sections, which is about 0.3 fT/Hz^1/2 per 1/cm^3/2 for our system according to our model.