Tunable Atomic Magnetometer for Detection of Radio-Frequency Magnetic Fields

We describe an alkali-metal magnetometer for detection of weak magnetic fields in the radio-frequency (rf) range. High sensitivity is achieved by tuning the Zeeman resonance of alkali atoms to the rf frequency and partially suppressing spin-exchange collisions in the alkali-metal vapor. We demonstrate magnetic field sensitivity of $2\mathrm{fT}/{\mathrm{Hz}}^{1/2}$ at a frequency of 99 kHz with a resonance width of 400 Hz. We also derive a simple analytic expression for the fundamental limit on the sensitivity of the rf magnetometer and show that a sensitivity of about $0.01\mathrm{fT}/{\mathrm{Hz}}^{1/2}$ can be achieved in a practical system with a measurement volume of $200{\mathrm{cm}}^3$.

Figure 1

Experimental setup of the rf magnetometer. A 3.8 cm square aluminosilicate glass cell containing 2.5 atm of ${}^4\mathrm{He}$, 60 Torr of ${\mathrm{N}}_2$, and potassium in natural abundance is heated inside a double wall oven to 190 °C. The K atoms are optically pumped with a broad area diode laser. The transverse spin polarization is measured using optical rotation of off-resonant linearly polarized light with a polarizing beam splitter cube and two balanced photodetectors. A set of coils inside the shields creates the bias magnetic field.

Figure 2

Comparison of theory and experiment for the dependence of the Zeeman resonance width (points with error bars) and rf signal response (open circles) on optical pumping rate. The solid line shows a fit to a numerical solution of density matrix equations including effects of light propagation, and the dashed line shows the width predicted by Eq. (2). The inset shows the comparison of Zeeman resonances under different modes of operation: (i) (points) at low pump power and high magnetic field; (ii) (dashed line) at the same field with an optimal OP rate; (iii) (solid line) in very low magnetic fields, when SE broadening is turned off [17].

Figure 3

The spectrum of the rf magnetometer near 99 kHz. The sharp peak is due to a calibration rf field and the broader peak is due to magnetic field and pump laser fluctuations. The magnetic field sensitivity to an oscillating magnetic field is $2\mathrm{fT}/{\mathrm{Hz}}^{1/2}$. Optical noise measured in the absence of the pump beam (dashed line) corresponds to $0.7\mathrm{fT}/{\mathrm{Hz}}^{1/2}$. Inset: Angular sensitivity $\delta \varphi$ (circles) in the absence of the pump beam as a function of the probe laser power compared with the photon shot-noise limit (solid line).