Enhancement of the Signal-to-Noise Ratio of an Atomic Magnetometer by 10 dB

Nonlinear magneto-optical rotation (NMOR) is a sensitive technique for measuring magnetic fields. The polarization self-rotation (PSR) effect is a fundamental source of excess quantum noise that limits the sensitivity of this technique. In this paper, we study the influence of the PSR effect on the signal-to-noise ratio (SNR) of an NMOR magnetometer in the Earth-field range and find that the SNR highly depends on the quadrature phase of light with a magneto-optical rotation. We introduce a phase shift before the polarimeter to improve the SNR by controlling the phase. The implementation of the phase shifter results in an improvement in the SNR up to 10 dB, achieving a best sensitivity of 8 pT/root Hz with an ordinary cell (without a paraffin coating or buffer gas). Our research should be useful for high-sensitivity magnetic field measurements in a real geomagnetic environment.